Is 20W-50 too heavy for my 340?

You could probably run 60W nitro racing oil in the Phoenix summer heat without issues in a 12:1 340, and that enviroment is one of the few times I'd consider the 20W50... I have ran 20W50 in some other engines and could tell it absorbed a little more power than 10W40 and it made about 15 PSI more oil pressure.