Synthetic oil, oils which viscosity etc.

[Myblueheaven]

To anyone who knows or has knowledge on synthetics, or semi synthetics. I have a 4 cylinder 75 hp Yanmar diesel, high hours , but runs great still. I want to try synthetic but notice a viscosity issue in synthetics. They seem to advertise super low viscosities than regular oil? Yes? NO? I know from my automobile experiences, regular oil car engine viscosities but not what appears to be low weight synthetics. like 5W-30W, 3W-20W? I am used to car oils like 20-40 or for my high performance (runs hot when racing) 20-50W. What should I use for my 75 horse Yanmar sailboat 4 banger? She's 1/2 to 3/4 of the way to diesel heaven. I want her to run smooth, and not make her parts work hard( lower viscosity) but she is getting looser than a bangkok whore in tollerences as she get older so more viscosity may be needed to make up the differnces. I am in southern CA, and plan top go South( warmer climates). What does the community of users recommend?
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[paulmcquillan]

The viscosity very quickly changes from the lower easy-to-start (cold) number, and becomes the thicker (hot) viscosity number.

I've run both Amsoil marine Diesel 15-40 and Mobil Delvac 1 5-40 in S. CA. Universal 5432 is much happier than when I bought the boat. We do regular oil analysis by Caterpillar Labs at each oil change. Look at the commercial truck or commercial oil distributors for the Delvac 1 --- have not found it at an auto parts store so far.

[Maine Sail]

I've tried in numerous times on automobiles and had oil leaks within 2-3K. I did this with three Volvo's and one Mercedes all engines had no oil leaks prior and then all of a sudden multiple leaks. Synthetic is a great lubricant but it also finds any possible weakness and can make old engines leak. Use a good Diesel grade oil and change often. I use Rotella 15W-40 and change every 40 hours. A friend did the synthetic conversion on his Yanmar and had 2 oil leaks within two weeks one requiring the engine be removed to fix it.. If Rotella is good enough for the truck fleets it's good enough for boats. Now if you had a new just broken in engine with 100 hours I'd say do it but you don't...