Oilman

Hi Richard Yes, if you are unsure, it's best to drain and refill. Cheers Tim

Coolant and antifreeze is a topic we are asked about on a regular basis, and often causes confusion; types, colours, service life etc. Perhaps the most frequent cause of confusion is what the difference between anti-freeze or coolant is. Basically, they’re the same product (although the term “coolant” could just be applied to plain water; see below!) To help clear some of the confusion up on the more technical details of antifreeze and coolants we enlisted the help of Martyn Mann – Technical Director, Millers Oils UK - who has provided the information below. Not all antifreeze / coolant is the same! Coolant can be plain water; water is a very effective coolant but would not protect against sub freezing temperatures or protect against corrosion inside the engine. The use of antifreeze protects against both problems. Antifreeze not only suppresses the freezing point of your engine coolant, but provides good corrosion protection and increases the boiling point during use. Most commercial antifreeze formulations include a glycol (to suppress the freezing point and raise the boiling point), corrosion inhibiting compounds and a coloured dye (commonly orange, green, red, or blue fluorescent) to aid in identification. A 1:1 dilution with water is usually used, resulting in a freezing point in the range of minus 37 °C to minus 42 °C, depending on the formulation. There are two basic types of coolant available today dependent on the corrosion inhibitors used: ·inorganic additive technology (IAT) ·organic additive technology (OAT) Inorganic Additive Technology This is the traditional coolant based on inorganic additives and is called inorganic additive technology (IAT). It is a tried and proven chemistry that provides a fast acting protective film. The additives deplete and the coolant needs to be drained and replenished every couple of years. This type can be used on all mixed metal engines with components including steel, cast iron, copper, brass, aluminium and solder without any detrimental effect. Organic Acid Technology The newer OAT coolants work differently than the older silicate based IAT coolants. Aluminium and ferrous metals form a surface-layer of corrosion in the presence of moisture, even with the little bit of moisture in the air. OAT coolants prevent this metal-oxide layer that protects the surface against this corrosion. Inherent with their design, the OAT coolants last longer than the older traditional IAT coolants. This category of antifreeze cannot be used in systems containing yellow metals. A couple of questions and answers. Why are coolants different colours? Coolants/antifreezes are coloured so you can visually see them; colour intensity can be an indication of over dilution. The different colours are non specific to the different types of antifreeze. The manufacturer can dye the product any colour they want. The colour is no guide to the actual type of antifreeze type and the label should be read before use. What is best for performance use? It is always best to use the engine manufacturer’s advice. If engine contains yellow metals [copper and brass as in older vehicles] then the long life products based on organic technology should not be used. As a general rule, most modern engines require the long life organic antifreezes. Is there any advantage to using concentrate over pre-mixed coolants? None other than the user may want to use the pre-mixed product due to ease of handling or cost and visa versa. Can concentrate and pre-mixed coolants be mixed? A simple answer is that you can, however do not mix IAT and OAT antifreeze together. So, there we go. Hopefully this information has been useful, if you have any further questions not covered here please ask and I will try to get the answer. With thanks to Martyn Mann and Millers Oils. Cheers Guy and the Opie Oils team.

Glad you like it, but it is down to Martyn Mann from Millers who has been most helpfull, he is the guy that comes up with all the oils/blends. Thinking of doing one on the black art of coolants. Cheers Guy

Hi Al Go for a super DOT4, Motul RBF600 and 660, Fuchs Pro Race and Millers 300 Plus are good choices and ideal for track use. http://www.opieoils.co.uk/c-450-brake-clutch-fluid.aspx Cheers Tim

Cool, glad you like it. More to come. Is there any topics that havent been covered, but you would like covered? Cheers Guy

Brake fluid... Bit of a mystery topic! To help dispel some myths and for some good solid general info on the mysterious world of brake fluids I decided to contact Millers Oils up in West Yorkshire. Their Technical Director, Martyn Mann was on hand to give us some useful info… below is Martyn's article on brake fluids. There is a degree of confusion regarding the specification of brake fluid and this article sets out to clarify the situation. The Department of Transportation (DOT) classifies brake fluids to defined specifications. These specifications relate to their boiling points and chemical composition, both of which are important. All currently available brake fluids are covered by one of the following specifications; DOT3, DOT4, DOT5 and DOT5.1. The laws of thermo-dynamics dictate that the energy from motion is turned into heat through friction. A braking system only works efficiently if the fluid remains incompressible. If the brake fluid boils, it turns to gas, which is compressible and the braking system becomes “spongy” or in extreme cases fails completely. A brake system is not perfectly sealed and moisture can get into the system and be absorbed by the fluid. The effect is to reduce the boiling point of the fluid, which reduces the efficiency of the braking system, as described above. The DOT specifies two reference tests for brake fluids. * Dry boiling point - the boiling point of fresh fluid * Wet boiling point –the boiling point once the fluid has absorbed moisture (representing brake fluid after time spent in a real situation). There are two main types of brake fluids: * DOT 3, DOT 4, Super DOT4* and DOT 5.1 which are based on poly glycol compounds. * DOT 5, which are based on Silicone. Note the two types of fluid are not compatible and must not be mixed in a braking system. SILICONE BRAKE FLUID (DOT 5) Silicone based DOT 5 was originally introduced to give higher temperature performance over glycol DOT 4. Silicone fluid also has other advantages, it does not damage paintwork and it does not absorb water. However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG fluids. It is also more compressible than PAG fluids, which can result in a sluggish or spongy pedal. It therefore requires special design considerations in braking systems. Further, because it does not absorb water, any water remains as globules, which can pool in low spots in the system and cause corrosion. This water can vaporise when heated under heavy braking giving a disastrous effect on braking efficiency. DOT5 fluids are not recommended for motor sport applications. POLY GLYCOL BRAKE FLUIDS (DOT 3, 4 AND 5.1) Glycol based DOT 4 fluid is the current mainstream brake fluid, and you will see that the specification is considerably better than DOT 3 which it replaces. DOT 5.1 has higher specification still and is for fast road and occasional track day use. It has a similar spec to DOT4 for the boiling point (>260) but is a lot lower viscosity @-40C typically 900 centistokes (compared to 1500 - 1800 centistokes for DOT 4 and super DOT 4). Listed in the table below, are the minimum dry/wet boiling point specifications for each DOT level. BOILING POINT: DOT 3 - 205°C (dry) / 140°C (wet) DOT 4 - 230°C (dry) / 155°C (wet) DOT 5 (silicone) - 260°C (dry) / 185°C (wet) DOT 5.1 (PAG) - 260°C (dry) / 185°C (wet) Super Dot4 * - 300°C (dry) / 195°C (wet) (racing brake fluid) * Super DOT4: The main difference between DOT 4 and Super DOT 4 is the dry boiling point. Normal Dot4 is >260C whilst Super DOT 4 is more like >310C With thanks to Martyn Mann - Technical Director Millers Oils. Cheers Guy. Opie Oils

^^^ This is correct. Synthetic is a better choice, for a standard car 5w-40, but the 5w-30 you got will be fine, especially this time of year! All oils mix so it does not matter it you mix some semi with a full synthetic. Cheers Guy

Good choice. Cheers Tim

At this time of year, it’s beneficial to use an oil that has good cold start flow properties as it will get to the parts of the engine that need it far more quickly when you turn the key on those sub zero mornings. The "w" number which means winter is the key here and the lower it is the better cold start performance the oil will have. A 15w or 20w rated oil will struggle to get around the engine in very cold temps and we would strongly recommend using a 10w, 5w or 0w for better cold start performance. It is a fact that around 90% of all engine wear occurs on cold start because the oil is at its thickest. The colder it gets the thicker the oil becomes and this affects the rate of flow which affects the rate of wear. These numbers help to explain the oils thickness and therefore cold flow performance at various temperatures. Grade.................At 0C.................At 10C..............At 100C 0W/20.............328.6cSt...............180.8cSt............9cSt 5W/40.............811.4cSt...............421.4cSt............14cSt 10W/50............1039cSt...............538.9cSt............18cSt 15W/50.............1376cSt..............674.7cSt............18cSt 20W/50.............2305cSt...............1015cSt............18cSt Centistokes (cst) is the measure of a fluid's resistance to flow (viscosity). It is calculated in terms of the time required for a standard quantity of fluid at a certain temperature to flow through a standard orifice. The higher the value, the thicker the oil. Winters in the UK are fortunately not too cold but, below zero temperatures are regular features in some parts of the country. Compare the thickness of the oil at 0degC and 100degC and you will see the big difference. Just something to consider on those frosty mornings. The Opieoils Team.

Although the basestock of an oil will be a major determining factor in the lubrication quality of an oil, chemical additives play a major part in making sure that it does all that it is supposed to do. The chemical additive package of an oil is just as important to insuring the quality of a lubricant as is the particular basestock used. The chemical additive package of an oil is designed to perform a number of tasks and each task is performed by a particular type of chemical. The quality of the chemicals used and the manner in which they are blended plays a large part in determining how well the additive package does its job. As the quality of the additive chemicals increases, so does the price. In addition, proper blending takes a great deal of research. This requires much time and, again, money. Therefore, manufacturers will, of course, charge more for motor oils which contain a high quality additive package than those with lower quality additive packages. They simply can't afford not to. Each chemical within an oils additive package plays a different role in boosting the beneficial properties of it's host lubricant (basestock). The additive package must perform the following roles: IMPROVE VISCOSITY CHARACTERISTICS Basestock lubricants have a certain temperature range over which they will flow adequately. The wider this temperature range the better. Cold temperature starting requires an oil that will flow well at low temperatures. The higher engine temperatures of todays smaller, higher revving engines requires an oil that will perform well under high temperature conditions. Pour Point Depressants In order to improve the flow characteristics of a lubricant basestock at low temperatures additives called pour point depressants are used. Because synthetic basestocks have inherently better low temperature flow characteristics, pour point depressants are typically unnecessary. Therefore, they are normally only used in conjunction with petroleum basestock lubricants. Waxy contaminants within petroleum basestocks tend to crystalize in low temperature conditions. These crystalized structures absorb oil and increase in size. This leads to oil thickening and poor low temperature flow characteristics. Pour point depressants do not inhibit this crystallization, as is thought by many. Instead, the pour point depressants are absorbed into the crystals instead of the oil, thereby lowering the volume of the crystals in proportion to the volume of the free flowing oil. This helps maintain the low temperature flow characteristics of the base oil even when crystallization occurs. CHEMICAL ADDITIVES Higher quality petroleum basestocks have less need for pour point depressants because they have lower levels of wax contamination. However, complete dewaxing of a petroleum basestock is not very economical, so all petroleum basestocks require at least some level of pour point depressant. Viscosity Index Improvers As a lubricant basestock is subjected to increasing temperatures it tends to lose its viscosity. In other words, it thins out. This leads to decreased engine protection and a higher likelihood of metal to metal contact. Therefore, if this viscosity loss can be minimized, the probability of unnecessary engine wear will be reduced. This is where viscosity index (VI) improvers come in. VI improvers are polymers that expand and contract with changes in temperature. At low temperatures they are very compact and affect the viscosity of a lubricant very little. But, at high temperatures these polymers "expand" into much larger long-chain polymers which significantly increase the viscosity of their host lubricant. So, as the basestock loses viscosity with increases in temperature, VI improvers “fight back” against the viscosity drop by increasing their size. The higher the molecular weight of the polymers used, the better the power of "thickening" within the lubricant. Unfortunately, an increase in molecular weight also leads to an inherent instability of the polymers themselves. They become much more prone to shearing within an engine. As these polymers are sheared back to lower molecular weight molecules, their effectiveness as a VI improver decreases. Unfortunately, because petroleum basestocks are so prone to viscosity loss at high temperatures, high molecular weight polymers must be used. Since these polymers are more prone to shearing than lower molecular weight polymers, petroleum oils tend to shear back very quickly. In other words, they lose their ability to maintain their viscosity at high temperatures. Synthetic basestocks, on the other hand, are much less prone to viscosity loss at high temperatures. Therefore, lower molecular weight polymers may be used as VI improvers. These polymers are less prone to shearing, so they are effective for a much longer period of time than the VI improvers used in petroleum oils. In other words, synthetic oils do not quickly lose their ability to maintain viscosity at high temperatures as petroleum oils do. In fact, some synthetic basestocks are so stable at high temperatures they need NO VI improvers at all. Obviously, these basestocks will maintain their high temperature viscosities for a very long time since there are no VI improvers to break down. MAINTAIN LUBRICANT STABILITY Lubricating oils are not only prone to viscosity loss over time. They are also susceptible to breakdown due to contamination and/or oxidation which decreases the useful life of an oil. Additives are often used in order to inhibit the susceptibility of a basestock to this breakdown over time. Detergents and Dispersants Contamination due to sludge and varnish build-up within an oil can often be one of the limiting factors in determining the useful life of an oil. If this build-up can be minimized and contained, the life of the lubricating oil can be increased. Detergent and dispersant additives are utilized for this purpose. There is some debate as to whether those additives considered to be detergents actually "clean" existing deposits, but at the very least they aid dispersants in keeping new deposits from forming. Detergent and dispersant additives are attracted to sludge and varnish contaminants within a lubricant. They then contain and suspend those particles so that they do not come together to form deposits. The more contamination within the oil, the more additive that is used up. Since synthetic oils are less prone to leave sludge and varnish, these additives are used up much more slowly within a synthetic lubricant. Some oils use ashless dispersants which are more effective at controlling sludge and varnish contamination than metallic dispersants. In addition, some ashless dispersants are actually long chain polymers that serve a dual purpose as VI improvers in multi-grade oils. Detergents are all metallic in nature. Anti-Foaming Agents Although necessary for engine cleanliness, detergents and dispersants can have a negative effect on the lubricating fluid within your engine as well. Sometimes, these oil additives can play a part in oil foaming. In other words, air bubbles are produced within the oil. These air bubbles, if not neutralized, will reduce the lubricating qualities of the motor oil. Anti-foaming agents such as small amounts of silicone or other compounds are used to control this phenomenon. Oxidation Inhibitors (antioxidants) Oxidation inhibitors are additives that manage to reduce the tendency of an oil to oxidize (chemically react with oxygen). They are also called antioxidants. The antioxidant reacts with the peroxides in the oil. These peroxides are involved in the process of oxidation. Reaction with the antioxidant removes them from the oxidation process, thereby lessening the chance of motor oil oxidation. Oxidation inhibitors also serve one more very important purpose. They protect against bearing corrosion. Bearing corrosion is caused by acids within your motor oil. These acids come from combustion by-products, but they can also be the result of oxidation. So, by inhibiting motor oil oxidation, antioxidants also protect against bearing corrosion. Corrosion Inhibitors Although antioxidants prevent the acids caused by oxidation, they do nothing to neutralize the acids caused by combustion by-products. Therefore, other additives must be used in order to keep these acids in check and to protect engine components from their effects. Some corrosion inhibitors are designed to protect non-ferrous metals by coating them so they cannot come in contact with acids within the oil. Other corrosion inhibitors are designed to actually neutralize the acids within the oil. Anti-Wear Agents Even with the best of oils there is always the possibility of metal to metal contact within an engine, however slight. Some oils (especially ester synthetics) will cling to metal surfaces better than others, but engines that are left to sit for any period of time may have very little lubricant protection at start-up. This is especially true in cold conditions when petroleum oils do not pump well. To minimize the engine component wear caused by these situations, anti-wear additives are used. Additives such as zinc and phosphorus will actually coat metal surfaces forming a protective barrier against wear. They do not eliminate the metal to metal contact. They simply minimize the wear that occurs during those instances. ALLEVIATE COMPATIBILITY ISSUES Some additives are included in an oil to deal with compatibility issues between the oil and certain engine components. For instance, there are certain types of lubricant basestock that will cause seals and gaskets to swell or to shrink. These effects have to be minimized. Sometimes basestock blending will alleviate the issue, but in other cases additives might be used. Depending upon the particular application the oil will be used for, some additives may be left out while others may be left in. For instance, in order to meet API SL fuel economy requirements, oils are now formulated with special friction modifiers. However, these friction modifiers can cause clutch slippage if used within motorcycle oils. So, motorcycle specific oils do not contain these friction modifier additives. When considered as a whole, Engine oils are comprised mainly of basestock fluids. Only a small percentage of the oil is comprised of additive chemicals. However, addditives can play as important a role as the basestock fluid itself. A high quality basestock blended with a cheap additive package will be poor oil. A high quality additive package added to a cheap basestock is no better. Of course, a motor oil as a whole is far greater than the sum of its parts. In other words, even a high quality basestock combined with a high quality additive package isn't necessarily going to yield a great oil. The company manufacturing the oil has to know how to correctly blend those basestocks and additives so that they perform well together. Cheers Guy

This is something different but well worth reading. It was written by our learned friend in Stoke and we think is of interest to any car or bike owner. Well…………! In The Beginning there was Carbon and Hydrogen. These got together in accordance with rules forged in the Big Bang (yes, really!) to make methane, one carbon atom with 4 hydrogens stuck on. A bit later, (only 4000 million years) other atoms started getting together and finally came up with Life, a self-reproducing chemical mix. The reproducing bit was quite fun, but after 600 million years even that gets boring. So, a more or less intelligent life-form invented The Car and the Motorcycle, the ultimate boredom cure. This was, and is, powered by the Internal Combustion Engine, which must have fuel. Methane is a fuel, which means it burns in air to produce energy, but unfortunately it’s a gas; a tank-full would propel a Honda 50 for about half a mile. But! Methane had not been idle since the formation of planet Earth, and had joined up with more carbons and hydrogens to make chains called ‘hydrocarbons’. Well, they weren’t called that at the time. They had to wait for a life-form to evolve that liked giving things names, and a hundred and 20-odd years ago chemists had to learn Latin, so they called the one with five carbons ‘pentane’, the 6-carbon one ‘hexane’, then ‘heptane’ then ….wait for it…. the 8-carbon one ‘octane’ and so on. (If we were naming them now the last one would be called ‘eightane’ so you would need 95 minimum REN for your engine.) All these things were liquids, very thin and volatile, and pure concentrated energy. The Hildebrand and Wolfmuller (rough 1894 equivalent of the Honda 50) now did 100 miles to the tank full. Unlike water, these liquids don’t stand around in lakes. They are hidden underground in porous rock so you have to drill for them. The old name was ‘petroleum’ meaning ‘rock oil’ but this was soon shortened to ‘petrol’. The petrol came out of the wells mixed with heavy oil, so it had to be distilled off in an oil refinery. Early on, the pale coloured stuff that evaporated easily and caught fire very easily was sold as internal combustion engine fuel. It was a simple as that. ‘Octane Number’ hadn’t been invented, but in modern terms this ‘light petroleum fraction’ was about 50 Octane. Now we all know that in the GCSE Science engine The Piston squeezes the air/fuel mixture, then The Spark Plug ignites it to produce The Power Stroke. The trouble is, with 50 octane fuel if The Piston squeezes too much the heat generated by compression makes the stuff Go Bang prematurely before The Spark Plug gets a look in, giving a Power Stroke with as much push as a fairy’s fart. This is why early engines couldn’t use compression ratios above 4 : 1, and 10BHP per litre was seen as hot stuff. Engines improved but petrol didn’t and even some time after WW 1 a touring 1000cc engine only turned out about 25BHP, and a hot-shot Sport version with the latest overhead valves would need a good tuner to get 50BHP. So finally some effort was made to stop primitive petrol going bang too soon, and a variable compression engine was invented for research. (The ‘CFR’ engine, as used for finding Research and Motor Octane Numbers, RON and MON, to this very day.) Early on researchers found that the bung in the CFR head could be really screwed down if a heavy liquid called ‘TEL’ (tetra ethyl lead) was added. This was really effective and cheap, and allowed the ‘straight’ petrol to be upped to 90 or even 100 octane, and a whole load of exciting high-power engines were designed around these fuels. This leaded fuel survived into the late 1990s, but much earlier an amazing discovery had been made. The shape of the petrol molecules was very important. ‘Octane’ if the ‘straight eight’ version with 8 carbons in a row had an ‘octane number’ of 25. It was only the mutant octane with 5 carbons down the middle and the others sticking out from the sides that gave the best results at high compression. (This special octane is still used as a standard for 100 octane. Proper name is 2,2,4-trimethyl pentane.) Today, ‘petrol’ is really a synthetic fluid built up from oil industry feedstocks. Very little of it is unmodified distillate from crude oil. It is tailor made to include the best compression-resisting molecules so that no poisonous and polluting lead compounds are needed to reach 95 or even 98 octane. Nothing much is added, apart from a touch of detergent to keep the engine top end clean. Quite a lot of petrol now has 5% ‘renewable’ alcohol as a planet-saving gesture, but this also improves the octane number (by about 1 ) so there’s nothing wrong with that. Anyway, if you have a motoring holiday instead of flying ComaJet, you are keeping that carbon footprint down….and paying too much tax as well…..but that’s another story. Fascinating stuff. Cheers Guy

Its quite simple, the engine is not designed to run on mineral based oil either. Engines to date are not smart enough to know what kind of oil you have put in it. They are designed to run on a visocisty and that ultimately is what the engine is looking for, quality is down to the owner. Manufacturers also list a minimum spec to be used, the idea of this is so the owner does not use an oil that is so crap it wears the engine out in no time, believe me these oils exist. As synthetic is better in every way to a mineral based oil it might explain why they are more popular, if you think about it mineral based oils are used for running in a new engine due to their inferior lubrication properties. Cheers Guy

Our look up guide just follows the stock recommendation for a stock car, in this case 5w-30. But yes, I do tend to recommend a 5w-40 over it and 10w-50 if heavily modified or used hard/track etc. Cheers Guy

For those of you going, we look forward to seeing you on Saturday. By the way, we have kept the Pre-Order Form open. - The Opie Oils Team

Just had a proper look at your car and with what you have had done, 10w-50 would be ideal. With what the mods cost, it had to be worth a bit extra in the cost of the oil to protect it. Magnatec just isn't up to the job of doing it properly

Last chance to place your JapFest Pre-Order. These Special Show Prices will finish Midday Tuesday 11th May. Dont miss out PLACE YOUR PRE-ORDER HERE >> - The Opie Oils Team

If the car is standard or close to it I would use a 5w-40 synthetic http://www.opieoils.co.uk/c-656-5w-40.aspx Out of those, the best ones are the Silkolene Pro S, Millers CFS, Motul 300V, Redline and Gulf Competition. The Motul 8100 X-Cess, Gulf Formula G, Fuchs Supersyn and Mobil Synt S are good, cheaper alternatives. If the power is up quite a bit or it's tracked a lot, go for a 10w-50 http://www.opieoils.co.uk/c-659-10w-50.aspx Both are really good oils, absolute top of the range ones, so go with any brand preference you have. Cheers Tim

Generally it's an overpriced semi-synthetic, but there are some new magnatec oils coming that are basic synthetics.Just have to see what they are like when they arrive with us

Best oil to use would be a synthetic, we recommend either 5w-40 or 10w-50 depending on state of tune and use. Cheers Guy

For most people Fed Ex do a much better job than all the other couriers that we've used and if you put a note on the order to leave the oil somewhere, they will. Or they can deliver to you at work, or a friend or relative. No need to let Fed Ex put you off. Believe me, some other 'reputable' couriers are a whole lot worse.

Anyone for some advice? Cheers Guy

These Special Pre-Order Prices will finish Midday Tuesday 11th May. Dont miss out on these Special JapFest Show Bargins and PLACE YOUR PRE-ORDER HERE>> - The Opie Oils Team

Opie Oils are offering unmissable collect in person special prices to all attending JapFest 2010 on Saturday 15th May at Castle Combe. Why not take advantage of our Special JapFest Show Bargains and PLACE YOUR PRE-ORDER >> you'll save £'s including carriage charges! With show special prices on 80 or so products from all of our major oil brands, place your pre-order today and collect from us at the show. Here are some of the JapFest special offers we've got - Silkolene Pro S Race @ £38 (5 litres - soon to be renamed Fuchs Titan Race Pro S!) Motul 8100 XCess @ £25 (5 litres) Millers CFS @ £38 (5 litres) Motul 300 V @ £19 (2 litres) Fuchs Titan XTR 5w-30 @ £15 (5 litres) See our JapFest Pre-Order Page for full details of all the products we have available at show special prices. Opie Oils are also proud to be sponsoring JapFest 2010. Our stand in the sponsors paddock area will host a selection of special cars driven by some of our sponsored drivers. Please note that we are not planning to take a lot of extra stock to sell on the day - if you would like to take advantage of our special prices, for collection on the day, pre-order here (no payment required at this stage)! We're looking forward to meeting as many of you as possible at what promises to be a great day! - The Opie Oils Team

Are you getting the noise all the time, or when hot or when cold? And what oil are you currently using? Cheers Guy

A 0w-40 should be okay as the oil is the same as any other -40 when hot -> a 15w-40 and 0w-40 are the same viscosity at 100C. The w stands for Winter and refers to the cold viscosity so the 0w flows quicker than the 15w when cold. One thing to remember though is that the cold and hot viscosities are measured on different scales as they are so different. An oil is always thinner when it's hot than cold, so a 0w-40 is no more likely to get past seals than a 15w-40 when it's hot.