Got the 400 back in my '67 after being rebuilt due to a spun bearing. Machine shop told me to run Penn Grade 20w50. Curious if that sounded right to you guys? I'm out here in CA.
Also a buddy of mine recommended adding posi lube to my Muncie m20. I plan on using 85w90. Any thought/suggestions would be appreciated.
Never heard of adding posi lube to a 4-speed. New one to me. I would not, just go with the 85w90.
Oil selection is often an opinion. It really depends on several things, namely your bearing clearances. Original GTO, Pontiac recommended 30 weight oil. 15w-40 and 20w-50 are racing oils. I have used them all in my older cars, and personally never seen any differences or problems. Those of us with "fast cars" always used 20w-50 racing oil because we had racing cars, right?
In summer heat, I feel the 15w-40 might be a good choice only because heat thins the oil. In winter, I'd use 10w-30, but I am originally from New England where it gets cold for a longer period of time.
I think oil choices are up to the individual. What you do want is an oil with zinc in it, unless you have a roller cam set-up. I assume by "Penn Grade" oil you mean Brad Penn oil? It has the correct zinc in it for older engines - so it is a good choice over adding zinc through an additive. You can go with 20w-50 oil if he recommended it. I might question him and see of 15w-40 would be ok. Thinner oil gets to places better as it is not as thick, and you want oil all over that engine. So I would ask him just so you don't have a peeing contest with him should anything go wrong with your engine and him use the oil weight as the cause.