Climatic conditions have the greatest effect on oil selection. I spoke to an engineer at GM once and asked him about the industry-wide switch to 5w30 oils.
After all, when you hear the claim that the newer engines are built with tighter tolerances, you really have to stop and think about it. The 3" crank in my 413 had only about .0020" oil clearance when measured on assembly. How much tighter could you (or would you) want to run a rod bearing???
During our conversation, he informed me the clearance issue was not factual and in fact, even modern engines have relatively the same clearances, inch for inch of journal diameter, as most older engines had.
He claimed the switch to 5w30 was simply because the factory wanted the vehicles ready for sale and use in the coldest possible climates and the reduced fluid friction of the 5w30 had an effect on CAFE standards.
He was also quick to caution matching the oil to the climatic conditions in areas where temperatures run very, very high and he personally preferred 15w40 because he felt it was very close to 'perfect' in terms of balanced viscosity over a broad range of operating conditions.
I have not experienced any startup noise on V8's with 20w50 oils. here in central California, we are over 100 degrees in the summer and never much below 34 degrees in the winter, so our oil selection criteria doesn't have to be quite so critical.
Thought this might be interesting...Robert
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