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Originally Posted by 99cobra
Brad what is the relationship between viscosity and heat produced by the engine? i.e does a higher viscosity oil (more friction) create more heat?
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I'm not sure that is an easy question to answer. I believe that if the oil has to low of a viscosity that you risk increased metal to metal contact, thus more friction and more heat (hence why none of us run a SAE 10 oil). Conversely, if the oil has to high a viscosity it increases pumping friction as the oil has to get squeezed through the small orifices, bearings, etc. This may cause increased fluid friction and HP loss.
A fluids viscosity goes down with heat. This can be seen on the product label sheets of most oils. Take the Series 2000 0w30 above. Its viscosity at 100F is 56.9 cSt, and at 212F it is 11.2 cSt. The stock engine is designed for an oil with a viscosity of about 10-11 cSt. When a 30 weight is cold, even as warm as 100F, you can see how much "thicker" it is and how much internal friction would be caused pumping that thicker oil through the motor. When 212F it is perfect.
Now consider the 20w50. Its viscosity at 100F is 128.1 cSt over double that of the 0w30 which is why you have to be very careful warming up an oil with this high an SAE grade. At 212F, its viscosity is about 18.3, still much "thicker" than the ideal. But as the oil warms up more its viscosity continues to drop. Somewhere around 270F it is just about 10-11 cSt, where as a 30 weight is down about 4 cSt - to thin.
I recommend evaluating this the other way around as I suggested above. Base the oil selection on the temps you are seeing and then re-evaluate. For my motor in the 70s to 90s a 10w40 is just about right.
