best type of oil for 71 340 engine

I run 20w50 in the hot summer most of the time and 10w30 for the best ET with the cold fall air at the track.

It's about how tight the engine is and how it was built ** BUT **

I would have to question why and how a 340 could ever be built to tight for 20w50.

Modern engines that use 5w30 oil or 5w20 oil *ONLY* are made very different then a typical American old school v8.

I run a heavy oil if in the hot summer or in a very hot climate.20w50.

If in a cold climate, i would run the lightest oil.10w30 in your 340.