This is a hot topic in another car group I belong in. If you have a flat tappet cam (stock style lifters, not mushroom or roller), you need an oil that is not made to be friendly to today’s catalytic converters. Without going into zinc and moly content and an insider’s thesis length paper about the best oil is actually a certain spec diesel engine oil,, Valvoline VR-1 in your desired thickness will do the job well. Skip the Zinc additives as too much of a good thing is not good either.
I bet 10w30 or 40 is what the car was designed for.
This is a screenshot of the flowchart for getting the best oils for non roller camshafts for another 60’s vehicle. Look at the Certification for CI-4 for diesel. The SL and SJ service ratings are the gasoline ratings to look for, not the newer ones.
If you have a roller camshaft, use the newest spec and best base oil you want to pay for.
if i cant find 10w30, i will use 15w40. but then i live in southern arizona where it is tough to be a minister, because in the winter you cant promise us heaven, and in the summer you cant threaten us with hell. i dont know where you live OP, but base your oil weight choice on your normal weather conditions, and use a good synthetic oil as well.
I have used Castrol 20 W 50 on my 65, used it for years in a 64 Fairlane 289, and in FE and ybocks. I live were it is hot and never really cold. North I would use 10 W 30.