Quote:
Originally Posted by Calum
Hey, thanks for the much more complete explanation. First off a disclaimer, I've lost a lot of the finer details in the almost decade since I studied this.
Basically what I was trying to get at was that with the periodic heavy loads that most of us will be subjecting our engines to, an oil with a better VI would be more appropriate. Also, by maintaining an oil with similar cold properties you'll ensure the oil stays within the designed operating range of viscosity, even at normal operating temperature. But, the oil won't be as proned to go out side the designed range of viscosity when we start beating on the car.
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Glad the info was of help. I can get carried away in the detail having been a formulator for over 30 years.
As you have said, higher VI, simply put, is good. However VI is also a complicated subject when you layer in the effects of temporary and permanent viscosity loss as well as any fuel dilution during use of the oil.
Just for clarity sake when trying to understand low temperature performance what the numbers in the table are telling us is that a
0W?? oil is supposed to guarantee that the oil is thin enough to allow an engine to crank fast enough to start at -35 deg C and thin enough to properly pump at -40 degC.
5W-?? oil is supposed to guarantee that the oil is thin enough to allow an engine to crank fast enough at -30 deg C and thin enough to properly pump at -35 degC.
You want the oil to be thin enough to pump at 5 deg C lower than the oil being thin enough to allow the engine to crank fast enough to start to ensure that the oil gets to all the places its supposed to be!
Some may say it never gets that cold where I live! However the lower low temperature viscosity results in reduced oil drag in the engine, improves oil flow and reduces fuel consumption. Also increases power.