Been boating 35 years. I have owned 6 4-stroke boats all with marinized car engines (Mercruiser V-8 or Mercury Racing V-8) two of which I still own. I only use Mercruiser 25W-40, originally the dino was all they had but now I use the semi-synthetic. I change the oil and filters no more than every 25 hrs on the Racing engines and every 50 hours on the standard ones. I've had two outboard 2-cycle boats one of which I still own. I only use Quicksilver Premium Plus synthetic blend. Never, ever an oil related issue. Ever. Why would the manufacturer shoot themselves in the foot.

BTW there are numerous oil threads on OSO with advice all over the map. Just use a quality multigrade with a low startup viscosity (say 10-25W) and with operating temperature viscosity say 40W. Do NOT use single grade except in the summer. In any event it is always best to pick a quality oil and stick with it.