Engine Oil Viscosity

 

Viscosity is the most misunderstood aspect of oil and yet it is the most important.

 

Viscosity is the force required to shear (break) the oil at a certain speed and temperature. Oils work because they have viscosity; the drag of a rotating part pulls oil from a low-pressure area into a high pressure area and “floats” the surfaces apart. This is called “hydrodynamic lubrication” and crankbearings depend on it.

 

Oil must be capable of flowing at low temperatures, so that it gets around the engine in a fraction of a second at start-up and must protect engine components at high temperatures without evaporating or carbonising and maintain adequate (not excessive) oil pressure. Many people think that the thicker the oil, the better the protection, but if the oil is too thick, it will not flow properly, leading to reduced protection.

 

The numbers on every can of oil indicate its performance characteristics when new but there are many misconceptions on what these numbers actually mean.

 

For multigrade oils you will see two numbers (for monograde oils only one). The first is followed by a “w” and is commonly 0, 5, 10, 15 or 20. The second number is always higher than the first and is commonly 20, 30, 40, 50 or 60. The first and second numbers ARE NOT related.

 

The “w” number (0, 5, 10, 15 or 20)

When multigrade oils first appeared, a low temperature test called “w” (meaning “winter” not weight) was introduced. Using a “Cold Crank Simulator", the test measures the oils ability to flow at low temperatures. ALL oils are THICKER at low temperatures than at high temperatures but the lower the “w” number, the quicker the oil will flow at low temperatures.

 

The second number (20, 30, 40, 50 or 60)

This number is known as the SAE (Society of Automotive Engineers) number and is measured in “Centistokes” (cst) at 100C. 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.

 

An oils cst at 100C determines its SAE rating within the following parameters.

SAE 20 = 5.6 to less than 9.3cst

SAE 30 = 9.3 to less than 12.5cst

SAE 40 = 12.5 to less than 16.3cst

SAE 50 = 16.3 to less than 21.9cst

SAE 60 = 21.9 to less than 26.0cst

 

ALL oils labelled 40 must fall within the SAE parameters at 100C so everything from a monograde 40 to multigrade 0w-40, 5w-40, 10w-40, 15w-40 or 20w-40 are approximately the same thickness at 100C.

 

Some oil companies label oils as SAE 35, 45 or 55, but as you can see from the above figures, there isn't a SAE 35, 45 or 55. This "could" be because they are approximately on the boundary of the two grades, but as we don't deal with any of those I can't really comment further.

 

Summary

 

Cold start.

A 5w-40 will flow better than a 10w-40.

A 10w-50 will flow better than a 15w-50

A 5w-40 is the same as a 5w-30

 

At operating temperatures.

A 10w-50 is thicker than a 10w-40.

A 15w-50 is thicker than a 5w-40

A 0w-40 is the same as a 10w-40

 

If you look above, you will see that the figures quoted do not indicate at all as to whether the oil is synthetic or mineral based... Well except for 0w oils as synthetic PAO basestock is required to acheive this viscosity.

 

Generally the oil you use should be based on the manufacturers recommendation found in the owners manual, but then modifications, climate and the type of use can affect that recommendation. If you are unsure of what is the correct recommendation for your car and would like to know more please contact us here oilman@opieoils.co.uk

 

With thanks to John Rowland of Fuchs/Silkolene

 

Cheers

 

Tim and the Opie Oils team

OK so my car has been standing for sometime thou i have change the oil a couple of times due to the engine out and new tubs.i did have fully synthetic oil in after the engine had been standing for a while but to me it sounded to tappy and proberly due not been able to give it a good run.after some sticky lifter treatment and some 10/40 semi it sounded much better and normal and queit.so a tt man with new tubs and 80.000. Miles what oil do you recomend.im going to be driving with cauton for a while until i now theres no loose oil pipes or leeks then ill properly change again and drve the norm.fast but with respect you understand

Back to a semi or full?

Personally I think synthetic is a better choice, what one were you using before? If the car is standard or near standard then I would look to use a good 5w-40 synthetic for all year round use. Plenty to have a look at here http://www.opieoils.co.uk/c-656-5w-40.aspx

 

Cheers

 

Guy

5W.30 FULLY SYN I think. Ill try 5w.40 whats the best in your opion?

The best are the ester based synthetics, but they are not cheap. Have a look at the Fuchs Titan Race, Motul 300v, Millers CFS and the Gulf Competition here http://www.opieoils.co.uk/c-656-5w-40.aspx

 

Cheers

 

Guy

And for a forged engine with Wisseco piston, which oil is better??

 

Thanks

It depends on oil temps, but probably a 10w-50.

 

http://www.opieoils.co.uk/c-659-10w-50.aspx

 

The Nanodrive CFS is, on paper, the best oil available, but the Pro S and CFS are both really good oils, so all are good choices for your car.

 

Cheers

 

Tim

is the stock car ment to use semi syn

You can use a semi, but a synthetic will give better protection.

 

Cheers

 

Tim

wasn't the engine designed to semi ?

A semi would have been the standard/decent choice when the engine was first made, but a synthetic is better for it.

 

Cheers

 

Tim

Hi there Oilman,

Ive always used the pro s 10w 50 for the zed http://www.opieoils.co.uk/p-1145-silkolene-pro-s-10w-50-ester-synthetic-oil-for-high-performance-engines.aspx

ever since I had the engine rebuild and has been a great choice,

I appreciate im going off topic from the zed but would you recommend the pro s 5w 40 for my 2005 Porsche Carrera s 33k miles 3.8 engine

kind regards

Dewi

Hi

 

Yes, the Pro S 5w-40 is ideal for the Porsche, one of the best you can have.

 

Cheers

 

Tim