Hey,
Personally I don't believe there is any ADVANTAGE of using synthetic oil in a marine diesel. I also don't believe there is anything wrong with it either.
Here is the manual for the Yanmar 4JH4-HTE-1:
Use an engine oil that meets or exceeds the following guidelines and classifications:
API Service Categories CD, CF, CF-4 and CI-4
SAE Viscosity:
10W30, 15W40. Engine oil 10W30 and 15W40 can be used throughout the year
Here are the specs for Shell Rotella T (not synthetic) 15W40:
API CJ-4, CI-4 PLUS, CI-4, CH-4, CF (and a whole bunch more).
So Shell Rotella T is clearly allowed by Yanmar
Here are the specs for Shell Rotella T6 (Full Synthetic):
API CJ-4, CI-4 PLUS, CI-4, CH-4, CG-4 CF-4, CF (and a whole bunch more).
So I don't see how Yanmar could void any warranty work on engines that used Rotella T6 full synthetic.
Again, I don't think that a marine diesel would benefit from synthetic oil, but I don't see how it would hurt.
Barry
Yanmar, who I suspect make more marine diesels than anyone else, will void their warranty on new engines if their oil recommendations aren't followed. The problem isn't a lack of lubrication with synthetic oil, it's that synthetics in cool running marine diesels glaze the cylinder walls and build up a very hard deposit on the edge of the piston crown that will score the cylinder. This subject has been researched fully and the results are easily available. However, don't take my word for it, read your engine's manual or get in touch with the maker. Or, to find out for yourself, run your engine for about 30 hours with a full synthetic and then check your oil consumption.