ive been thinking... dangerous i know but here goes....

If an oil is fully synthetic, ie completely man made then it must contain no real oil ie the crude black stuff that comes out the ground.

so if it doesnt contain real oil, then what is it made of? nelson over to you.

Fully synthetic oil is not man-made. All oils come from naturally made crude oil. Oil made from minimal refining is called mineral oil. Mineral oil which has undergone some chemical processing (I don't know what exactly) is called semi-synthetic. Oil which is severely processed and refined is called Fully synthetic.

 

The advantages of this processing/refining are to make the oil work better as a lubricant, age slower and they tend to be thinner which means better fuel economy and power.

 

Fully synthetic oils are not always recommended for older engine tho' as they can cause problems becasue the engines were designed to cope with ordinary (mineral/semi-syth oils). New and high performance engines (but not really old ones) are usually OK with fully synthetics.

Also - oil gets thinner as it heats up -this means it is relatively thick when cold. Fully syth oils have lower viscosity-temperature sensitivity, ie. their viscosity changes less with changes in temp. -this allows the oil to be thinner when cold which means better fuel economy.

 

The ideal lubricant would be iso-viscous (viscosity doesn't change with temperature) - fully synth oils are closer to this ideal than semis and minerals.

 

I think the better base-stocks used in fully synth oils allow the additives to perform their jobs better - friction modifiers/antiwear Zincs etc. Chemical species are also removed which would cause relatively rapid oxidation of the oil which leads to significant thickening of the oil, deposits, wear etc..

After I added zMax to my 90NA almost-stock, it began leaking aroung the gaskets and seals. When I switched to fully synthetic, the leaking became serious, even with stopleak (to recondition the gaskets). Since my engine has about 150,000 miles on it, should I switch back to semi- or normal? What additives should I be using?

 

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--Maxx

I think that its 2 of the 5 base 'dopes' (they were referred to as) are natural the rest are synthetic then the oil is classed as synthetic.

3-4 of the 5 base dopes are natural the oil is semi synthetic.

and so on, thats how I recall things working. As mentioned its just the processing the oil goes through. Fully synthetic has more man synthesised dopes than straight mineral oil.

Castrol are real ****s, as they only list on the bottle as being 'synthetic engineered', when what they should be saying is semi synthetic, that pissed me off rather a bit. but hey, c'est la vie or something.

 

As for the leakages, mineral oils can leave 'deposits' which can then be washed away by a more pure synthetic oil, and once the leaking has started you cant really stop it apart from changing the gaskets.

and if possible you should change the oil type as little as possible, i.e. get one oil and stick to it, either should be fine really a good semi or fully synthetic changed often.

Mobil 1 motorsport 15w50 I quite like and nelson recommends the shell helix range.

As for oil additives, just say no! hehe, really if they were any good, manufacturers would put them in the oil in the first place, as im sure its not beyond the skills of an oil manufacturer to develop said additives and include them in their own package, but they dont sell them, i think that says it all really. theres so many more arguments for not using 'additives' but ill not bore you, just dont use them, you are better off spending the money on a better oil!

I was reading a RX-7 site a few nights ago and there it said dont use Full Synth oil for 2 reasons.

1 it doesnt really allow carbon deposites on the wrankle which is needed to create a seal.

2 Full synth doesnt burn, so if any oil, how ever small, gets into the head, it will not burn with the fuel and end up going out of exhaust onto the cats. However semi synth and normal engine oil will burn and so only fumes will go out of the exhaust.

 

Dont shoot me, this is just what I read.

 

Stuart

Stuart

I dont think 2 is entirely true. In the combustion chamber even synthetic will burn. It is just that it needs a much higher temp than standard oil to do so. Hence the reason its especially preferred on turbo cars cos it is less likely to burn down to coke residue on the turbo shafts and bushes and wreck seals if the engine is switched off too soon after a roaster of a run. And as we all know, coke on your shaft does no bush or seal any favours!

Obviously its higher temp withstand is also important on highly loaded bearings and piston rings.

Willie

If your talking oil requirements for a 'boinger' engine you should really not consider the RX-7. The RX-7 'Wankel' type engine requires altogether different oil properties. In an RX engine oil is injected into the engine(it's needed to form a seal between the combustion chamber and the 'apex' seals) and burnt during the combustion process. The use of a synthetic oil could lead to unwanted debris building up in the combustion chamber. It's recommended that a 20/50w mineral oil should be used in these engines and give them a good blast every now and then to get rid of carbon deposits.

Also if anyones interested, if an RX engine won't start it's usually because it's flooded and the oil has been washed off the apex seals. It's a bit of a rigmorole to get it started again but it not usually a 'blown ' engine as reputation has it.