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.

no, 5w-40 lubes better upon ambient temp engine startup that 10w-40 and much
better than 30w or 40w does.

however, the difference between 5w-40 and 10w-40 isn't all that great. In
past, 5w-40 didn't hold up as well under long service, and so was seldom sold.