I have a gator tail surface drive mud motor and a 42” quest. My gator tail mechanic told me years ago to use Rotella 15W40. I use this in all mowers, generators etc.
Rotella is a diesel engine oil. Initially, the "zinc package" (to benefit camshafts) was removed from auto oils because of the catalytic converter on newer cars. But diesels don't have the 'cat', and so the zinc remained.
However, the newest diesel oils no longer have the zinc package either. So the recommendation from "years ago" is not the
hot ticket it once was.
The EPA is changing fuel and lubricating oils to cut air pollution. Many tips concerning
gasoline and oils that served our fathers well, no longer apply. In fact, a lot of those tips will actually do your engine harm now. It's a whole new ball game, and re-education on these topics is required.