Just talking small sailboat auxiliaries here (not talking big powerboat engines, whether pleasure or commercially rated, where one should just follow the engine manufacturer's recommendations for all fluids - this is always easy to due to the much larger bulk quantities to be purchased).
A few years ago I discussed engine oils for these with my favourite oil manufacturer's technical people both with respect to synthetic oils and also for the cases where the engine manufacturer specifies single
grade oils. Their recommendation, regardless of whether a multi or single
grade was specified by the engine manufacturer, was to just use whatever the oil company's current
high volume product was on the shelf for diesel
engined passenger road vehicles. Said that is always the best choice including for ease of purchase
. I suspect that they are correct and have followed that in my own case.
, for first fill I buy the box spare part oil to determine what it is if they do not otherwise fully specify the oil. From then on I just buy the equivalent from my favorite oil manufacturer, although often minimum pack quantity is 4 litre - enough for 4 or 5 refills for many small boxes. In my current
case oil turned out to be a multigrade synthetic gear
oil and that is what I use.
I make a habit of running the engine, well loaded, for at least an hour at least every 2 weeks all year round, and suspect that has more value than fussing about oil choice.