BOAT ENGINE LUBRICATION

Assuming all load and equipment condition factors are the same, phew!
Antiwear additive levels need to be compared(P ZN and any others)Type may be important.
What API Spec?
Viscometrics(rheology) need to be compared in depth.
What type of synthetics PAO, Esters versus what type of mineral oil, VI?. Mineral oil may contain bright stock(portion of viscous oil) which may be helpful under heavy load.
Like to see data sheet for both.

Synthetic doesn't mean better. Synthetic means more time without oxidation at high temperature. Performance at 100C depends mostly on the additives and there is almost no benefit from Synthetics...if changed at regular intervals.

I found that over and over where certain companies promote Synthetic, and it is not better unless the performance ef the engine is really high, i.e. high compression, high RPM engines. For your long stroke, high displacement for relatively low power engine, there is no need of a synthetic.