I think Maine has the real issue here. How many failures are due to oil related failures? Will synthetic help a motor last longer than one that has proper 100 hour or once a year changes? I doubt it, at least not in a sailboat. In a trawler that does a few hundred hours a week yes it might, but I doubt it will in a sailboat. What is the main cause for failures in a sailboat, I think from my years of watching the forums, (as I have not yet experienced one) it seems to be overheating and head gaskets that are the biggest downfall not bearing failures. An impeller breaks or intake gets clogged and the motor overheats. Often due to safety issues you are not able to turn it off in time to prevent damage. Perhaps some issues may be caused with rings and valve stems, but normally not fatal, just oil burning and extra smoke. I don't think the wear on these heavy duty low RPM engines is enough to justify the expense of synthetic as they are unlikely to see any benefit at all. Who really cares if the bearings are like new if what is likely to do the engine is a cracked head or block after an overheating incident.
I know I use synthetic on all of my personal cars. I had a Miata that was just shy of 200,000 miles when I sold it, but that is a very different scenario from a marine diesel. That was hard driven HIGH RPM use. I tracked the car, and auto crossed it so it would spend hours at a time at or near red line. I changed oil depending on use, about 8,000 miles of street use and after every track event. It did not burn or leak a drop of oil and I switched it to synthetic oil at around 70,000 miles. My BMW uses synthetic as factory recommended but I change it more than recommended. I drive about 18,000 a year and change it every six months. I just think 15,000 is to long to go without oil analysis. Since the analysis costs about the same as a change then it seems to me smarter to just change it because if it comes back as needing to be changed it costs twice as much (cost of analysis and cost of oil). Cars are a different animal, as they are not low RPM and light duty motors.
Sailboats with perhaps twice a year changes, is all they need for a long long life. Unless of course you have a Volvo, we all know they will blow up anytime!