This is timely, yesterday a rep came to my shop from Joe Gibbs Racing and explained the deal with the oils. He said the biggest problem with the earlier flat tappet motors is the lack of zinc because zinc messes with the catalitic converter. There is an API rating on the back of quarts of oil, the API is a spec that they have to meet, castrol API type SM is the same as valvolen API type SM etcetc, there is some proprietory differences, kendall makes thiers green ok? Type SM the government specs no zink. That is the stuff taht whiped the cams. The newest generation is API type SN, that stuff has a detergent molecule that finds the additive you put in there and washes it right out!
You need to find oil specifically made for you hot rod. I stock and use Joe Gibbs oil (it was coincidence that i had thier stuff in stock), they offer both synthetic and conventional for hot rods. Go to thier website and read up on the HR series(hot rod) HR-1, HR-2 are both conventional oils; HR-3 & HR-4 are synthetic.
He really got my attention when he said the newer blend of oils will remove any additive that we put in there.