I don't profess to be an expert on oil by any means but I've read several articles that advise when using a multi viscosity oil, after a few warm-up, cool-down cycles the viscosity modifiers break down and essentially turn the oil into a straight weight oil approximately half way between the listed viscosities. For example if you use a 10W30 you end up with close to a straight 20 weight viscosity oil. Is that factual? Beats me but I've read that numerous times in different automotive publications. Something to consider. Gene Schneider over at the VCCA recommends 10W30 year around. I have used 15W40 year around for several years with no adverse effects.

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