The "Mysteries of Lubrication" -- all ears.
Davis Modlin said:
The main reason for using a single grade oil is that the reversing gear uses the same oil as the engine. Also multigrade is made for the changing conditions that cars go through.( changes in altitude, temp, etc...). I don't think that a boat will have to go throgh these changes during any single season.
This makes enough sense... but I think you'd want to pay close attention to the "detergent" qualities of the oil. There was a whole explanation of lubrication in the John Muir "Compleat Idiot" book that comes to mind.
Now under normal circumstances a marine engine _should_ operate in a far more thermally benign environment on a boat but the issues of cooling failures can provoke some anxiety.
Davis Modlin said:
As for motorcycle oil, the only thing that I can cime up with is that motorcycles run at a much higher rpms than the A4. So the oil may not be suitable for a much slower engine.
Well, oils designed for motorcycles have the mixed demand-- transmission *and* engine-- (though I think there are some older Harleys that had a seperate transmission) so they'd fit the application; however, my VF500F was water cooled (with an electric motor to drive the radiator fan) and so the temp extremes weren't there.
As for RPMs and wear-and-tear... I *do* know that the most stressful part of an engines operation is the low RPM end since that's the end with the poorest oil pressure with the same valve loading on the camshaft... but motorcycles (especially the overhead camshaft varieties) are *especially* sensitive to lubrication failure.
In other words, I don't know... but I'm not sure anyone else really does either.
I now suspect that there's a lot of conservatism in boat owners, mostly driven to avoid an "experimental failure" given the high costs to replace, say, a "smoked" A4.
So perhaps a set of "advisories" (based on some kind of testing, even if done w/ dismounted engines) for oil selection (to fit various engines) would be of use... along with a fairly in-depth consideration of clean-up.
Right now I've only got an 8HP 2-stroke O/B which, well, bothers me at times. Granted, it doesn't sound like the Yamaha RD-350 it likely should (the ring-ding-ding-ding-ding sound being made underwater) but the cloud of smoke _does_ provide some discomfort. I eventually (i.e. "when I can afford it") be looking for a 4-stroke to cut down on my anxiety.