I don’t have a saildrive
myself, but was interested so I looked and it appears that Yanmar
specifies a 15W-40 oil
Now if true that tells me that it doesn’t have any special oil needs and they spec’ d what your most likely to have already as multi viscosity oil in a heartbeat case is unusual, cause they don’t often run at high temps.
Then again I don’t know if it means anything, but some high performance differentials for automobiles recommend against synthetics, but specify short oil change
If nothing else this ought to at least bump your thread, but gut makes me think any good engine
oil is fine, but change it often of course watching for it to turn milky.
as much to bump your thread and learn something myself