Phil - it could depend on the type of cam and lifters you are running. Also, if you know the builder the safest bet may be to speak with them. Depending on how they normally set up clearances and
oil restrictors and the type of pump they install they could have some strong preferences.
On my Keith Craft FE, which is a mild to moderate build, they recommended Joe Gibbs non-
synthetic 15W-50 (maybe it's 20W-50, can't recall).