Most engines in good shape (tight-ish clearances) will maintain good pressure with 10w-30. That is what I use on my BB stroker and it has great pressure.

You may be able to run a lighter weight (ex 5w-20) but I think that may be pushing it for these older motors. Techincally if it can still build pressure, it should be fine but that oil is like water IMHO. For the few HP you may pick up on the lighter oil and the potential for losing pressure if the clearances open up any, I wouldn't go below 10w-30.

As for what brand, you will get a ton of opinions on that one here but you asked for weight only so I suggest 10w-30