I concur with R Pope. I've got a 351 GT-40 in a ski boat and the owners manual says run straight 40W.

I'm surprised the oil cooler is not more effective than that. Where are measuring the oil temps?

Those temps are not too bad, particularly for synthetic oil. I would consider getting some hours on it (50-75?) and then switch over to a 20W-50 synthetic such as Redline or Royal Purple.

Nice engine and pics. That must be a scary blast on the water! I'll bet it will fly around most everything (except the gas station).

Kitz