All inboards I know running use Rotelle 15-40w with many , many hr's of use with no problems. But I been toying with the Idea of switching to synthetic and double my change intervals by just changing filters every 100 hrs.
There is a additive package that was added to the 4 stroke oil and it has a designation 4S or something like that.
But just think of all the years that 4 stroke motors ran on the water before that was available. My '03 Suzzy's do not call for the 4 stroke Marine oil, as it wasn't even available at that time. This is according to the owners manual.