The oil was never an issue- - - -General Motors invented the whole thing to hide a design malfunction on certain engines. An EGR passage near the #6 cylinder on small block Chevy engines caused the cylinder head to overheat and coke the oil around a pushrod- - - - -which caused the pushrod to jam and wear out a camshaft lobe. 10W40 oil had enough polymers to do that before the warranty expired, although it would happen eventually with any oil, regardless of weight. GM invented the "Don't use 10W-40" myth to avoid replacing engines under warranty. I was teaching auto mechanics in a Tennessee state trade school and attending GM training center seminars every year while that was happening- - - - -mostly on high performance 305 Chevy engines with QJ carburetors and aluminum intake manifolds. I've had to drive stuck pushrods out of cylinder heads with a hammer and a punch at times. GM wouldn't identify a specific brand of oil that was common to most of the failures, but there was a "Q" and an "S" in their logo!
Jerry
