All of the above may be total BS, but I did get it from a reliable source, I
think, but unfortunately that was so long ago that I don't remember who or
how reliable that source might have been.

I think its BS.
My Mercruiser manual says to use 5-40.

What the hell is that? so 10-40 isn't good enough? Real reason is to
cause F.U.D. to discourage boat owners from doing work themselves.

John C.