Back in my youth, with my very first OB, I used to buy **** oil at the local **** store. It was cheeeeeeep.
Then I had a problem. I took the engine to a local Merc dealer during the Winter break. The service manager took pity on a poor college kid and offered to diagnose it for free.
After 1/2 hour, he said without hesitation, you have been using that crap **** oil, haven't you. I couldn't deny it.
That was 55 years ago. Since then I use Merc oil in a Merc and Rude oil in a Rude. Haven't had a problem since.
Now I realize that my procedure may be un-necessary. However, if you pay 60 grand for a new boat, what's the sense in trying to save money on service, when the cost savings isn't even enough to buy a six pack of beer?