Quote:
Originally Posted by Calum
As described, the number before the W indicates the cold viscosity and the number after the W indicates the hot viscosity properties of the oil. Given that this car calls for an oil that has low viscosity at low temperatures, 0W, I would stick with something that also is a 0W. But the Higher number is a little ambiguous. The lower the number is, the more the oils viscosity drops as it gets how. Changing to an oil with a higher number after the W will give you a more temperature stable oil more then anything else. The viscosity at normal operating temperature wont be effected much, but will remain in the correct operating range for the engine for a higher temperature.
I'll be running Mobile 1 0W40.
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Sort of. The number after the W does not represent a rate of change of viscosity with temperature.
It does however represent a range of viscosity values at temperatures and conditions that oil producers and engine manufacturers have jointly agreed are relevent to engine operation as it relates to oil thickness. To be listed as say a "30" in an ?W-30 requires that the oil fall within a range of viscosity values for kinematic viscosity of 9.3-12.5 cSt at 100 deg C and have an HTHS(high temperature high shear) dynamic viscosity of greater than or equal to 2.9 mPas at 150 deg C. ?W-20 would be 5.6 to 9.3 cSt and HTHS greater than 2.6mPas.
HTHS dynamic viscosity requirements actually relate to temperatures and oil shear rates in connecting rod bearings as a mechanical point of reference.
http://www.tribology-abc.com/abc/viscosity.htm#dynamic
Scroll down to the table for
SAE Viscosity Grades for Engine Oils1 — SAE J300 Dec 99
The industry standards in the above table show the minimum requirements that oil producers and engine manufacturers use for categorizing the flow characteristics of engine lubricants. Engine designers can then design engines knowing what properties to expect from products based on their viscosity grade.
Lower HTHS values have demonstrated better fuel economy, but because the oil is thinner at high temperature, care has to be taken to ensure engines recommending these viscosity grades are appropriately designed to use them. If an engine has been properly designed to use lower viscosity engine oils adequate film thickness should be available from the recommended viscosity grade under conditions specified by the engine manufacturer.
Now if you plan to subject the engine to higher loads and temperatures, than what the manufacturers suggests are normal, as the owners manual in this case suggests, one might want to consider an oil with a higher number after the W. Conceptually, there was however, an old rule of thumb which suggested that you have more potential for wear from an oil that is too thick at low temperture than you would get from one that is too thin at high temperature.
This can get to be a complicated subject rather quickly!