for any engine, most conscientious owners will use either cheveron delo 400 or shell Rotella T.. in a 15/40wt. changed regularly.
its kind of a personal preference which brand you use.
the synthetics are useful in some engines, but are a waste of money in a boat engine where the temps and rpms stay fairly low without spiking.
they are great in extreme temps and high rev engines, but offer us nothing.