Sorry I crated so much controversy. I thought the OP was a simple question, but obviously, opinions run strong and varied on this subject.
I was going to inquire about oil filters, but that may create even more controversy. I've used up the AC PF31's I had, and plan to change to the Purolator L30005. Both of these are the long style, and I know the Purolator has an anti drain-back valve.
My wife has an identical Tripower 434 Whitmore engine in her car, so the correct oil & filter choice is doubly important to me.
That being said, I've run Brad Penn 20W50 for the 7000+ miles I've put on my engine and my wife's with about 4000 miles since Dan Whitmore built them. I wish he was still with us so I cold discuss this with him, the most knowledgeable person I've known about Pontiac engines. I do remember he provided 20W50 for the initial fill.
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BONESTOCK GOATS
'64 GTO Tripower Hardtop (Wife's Car)
'64 GTO Tripower Post Coupe (My Car)
'99 Bonneville SE Sedan
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