Shell Rotella T 15W/40 is what nearly everyone uses these days. Delo 400 is another good diesel oil. They are both available at Wal*Mart or any autoparts store.

I would not run a synthetic as Westerbeke still advises the same change interval with either synthetic or conventional oil which makes a synthetic more expensive than is necessary. These engines will go 10k hours + on conventional oil if well maintained and run properly.

Peter, the C rating is essential, since that is what rates it for use in diesel engines, compared to gas engines. Synthetics, in addition to what Maine Sail notes, have a tendency to negatively affect seals. Use dinosaur oil.

Does anyone use anything other than Rotella T 15W-40?

Have to say that for $14 at the local Wal-Mart, seems like a no-brainer.

Yes, sometimes I get four quarts, sometimes I get a gallon, usually different dinosaur oil available on sale, C rated, of course.

Storing the remaining quart bottle is a lot easier than storing the remaining gallon with less than a quart remaining in it. I don't ever have to fill up with oil between changes.

Costco has Delo by the case. Well, heck, they even have dental floss by the case. Ask me why we have a three car garage and no car can fit in it.