The GM 350 diesels, BTW, were junk for many years ...
Back about then, the Director for Engine Research at the GM Research division claimed that the engines were good enough for the intended use, and that 100% of the engine failures that were inspected at the Research labs were caused by improper use of 10W40 engine oil, which was specifically NOT APPROVED for use in that engine.
He stated then, and the few operating manuals I checked agreed, that GM had NEVER listed 10W40 as approved for use in ANY GM Engine.
He made a very plausible argument for why it wasn't good - for any engine. (A decent explanation would be "longish.") And he was one of the first technical people I can recall who put the blame on "the big oil corporations."
At least one of his ideas seems to have taken hold, but the "public" still thinks that when the book says 10W30 then 10W40 is better because "it's an even bigger number."