I hear this subject often and I always read, that you should stick with the manufactures recommendation.
I just read another "letter to the editor" type article. The question was wether or not he should change to a synthetic oil in his 350 mag that has 900 hrs on it. And, added that it was a fresh water boat.
The tech replied, evidence by the 900 hours on the engine, I would say that what you are doing is fine. Stick with what you are using. Quality of the oil and how often you change it are the important factors. He also added that if you think your engine is getting tired or you see lower oil pressure when it is warmer, than definatly run with an SAE40 all the time.
I, myself have heard that synthetic, and marine engines don't really work well with each other.