Quote:
Originally Posted by The Gritz Carlton
I use 10W30 Synthetic year round in everything except the diesels. Our weather here doesn't get so cold you need the super thin stuff at startup. Part of the 5W promo is for added fuel economy...it helps with their "cafe" numbers.
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This touches on the reason for the lower viscosity number (e.g. 5W). There was a time when the best you could do was 10W. In super cold temps, oil couldn't flow quick enough, resulting in main journal, cam bearing and lifter wear. The lower viscosity numbers helped the oil flow more quickly, coating the surfaces before heat build up, which happens quickly in the rotating mass, burned the bearings. The winters are long in some places. If that scenario plays itself often enough, you're tearing the motor down to replace bearings.