Use a good quality diesel
rated oil per the manual. For my trucks and tractor, I have used Delo, Shell, and JD. Mostly Shell 5w-40 and lately JD 0x40 oil. The Shell 5w-40 is a semi synthetic and the JD is a full synthetic. I switched to JD because it is a slightly better oil and a few years ago, was cheaper in a 5 gallon bucket than the Shell in 1 gallon jugs. I got slightly better MPG with the 5w-40 and 0w-40 oils compared to 15w-40 oils. Using UOA(Used Oil Analysis), I run the oil to 12,000-14,000 miles vs 5,000 per the truck manual.
of the JD oil has gotten higher the last few years so I might go back to using the Shell 5w-40 oil since I can get it on Amazon.
A big advantage to me is that 5w-40 or 0w-40 oil starts MUCH easier in cold weather
than 15w-40 oil. The first set of numbers is the weight of the oil when cold. In the past, I would use an engine block heater prior to starting my truck engines. The block heater is no longer needed with the 5w and 0w oils. The block heater cost me about 50 cents a day and quickly adds up and pays for the higher cost synthetic. I had no problems starting my truck the last few weeks even with the temperatures down to 6F.
Not sure if you need this cold weather
ability on a boat in OR. Personally, I would be more likely to use the a xxw40 oil than xxw30 oil but I would check the temperature range in the manual.
FYI, my JD tractor has a Yanmar engine which has been flawless. My tractor dealer, who are really good people, told me that the Yanmar was a great engine with very few problems. The JD engine that replaced the Yanmar was not so reliable. My Yanmar uses no oil what so ever.