I've been running full synthetic 20W50 oil in my 454 mag since it turned 100 hours. Now have 300 hours on it without any problems at all. Doesn't burn much oil at all even with Forged pistons.
One thing I have noticed versus the strait 40W conventional oil I used before is the oil pressure is higher with the synthetic under all conditions, even after a long hard speed run.
I think the oil pressure remains higher due to 2 factors. 1) The oil stays cooler), 2) the synthetic oil doesn't breakdown as fast under the extreme conditions a boat operates in.
I would wait a while before converting to synthetic if you decide to convert. You want to make sure you motor is broken in completely. Not sure of the recommended hours but I waited until 100 hours.