After a lot of semi-educated guessing, GM figured out the problem causing the flat cams was the EGR valve. The EGR passage ran through the cyilnder head right between #4 and #6, and the heat of the exhaust gas would cause the valve guides to tighten up. That overstressed the cam lobes. It would also "coke" the oil in that area to the point I've had to drive pushrods out of solid chunks of carbon lodged in the cylinder head.
In the mid-80's, GM would not warranty an engine if it ran 10W-40 oil, because the oil-coking problem seemed to be worse with that weight range than any other. The refused to specify a brand name that was causing problems- - - - - coughcoughquakerstatecoughcough! They just told dealers to refuse warranty coverage for all engines running 10W-40.
The change to electronically-switched EGR's in remote locations from the cylinder head/intake manifold area pretty much cured the problem.
Jerry
