Quote:
Originally Posted by Jeff92se
Don't know why one would want to run 3 times the rated winter viscosity if they didn't have to. (and stress the oiling system w/o need)
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It is not 3 times the viscosity. 20W is the same minimum viscosity at 212 degrees F as 20 weight oil but not at the winter temperature rating. As an example with 15W oil at 5 degrees F you have the same rating as 5W oil at -13 degrees F. At 212 degrees F #15W is rated the same as 20W and single weight 20. These are minimum values. Maximum value ratios are a little different. As a comparison single weight 30 has a viscosity rating of about 25% higher than single weight 20, not a 50% increase that may have been guessed by many. 20W oil at its temperature rating of 14 degrees F has a viscosity of about 30% higher than 5W oil at is low temperature rating.
For this reason I would not use a higher oil than 15W in most northern places in the U.S. On the other hand its pumping capability is the same at -4 degrees F as 5W is at -31 degrees F. In other words if you can crank it, it will pump.