Synthetic is fine, and if your rings are sealing well (and oil stays visibly clean) you can run extended change intervals. Change the filter around 50 hrs and top off, then change oil at 100 hrs. Synthetic oil does not break down as quickly as dino oil. For a flat tappet cam you generally want phos/zinc levels over 1000 ppm. Stock pressure valve springs and 800 ppm is fine, I would be concerned with less though. More aggressive ramp rates on the cam and higher spring pressures and 1000-1200.
http://www.mobiloil.com/USA-English/...duct_Guide.pdf
http://www.amsoil.lube-direct.com/20...nc-oil-amsoil/
Basically for a stock marine cam with low lift, slow ramp rates, and low spring pressures you can run whatever oil you want and have no worries. If you upgrade the cam and valve springs, be more picky on what oil you run.
I am averaging a single oil change a year on both the boat and the tow pig... Next year it might be two oil changes on the boat while still one on the tow pig. Only running synthetic in two vehicles though, not the boat.
'86 Comp TS6M - Reborn 2016
Riding a HO Sports CX Ski