FYI. The fine folks at Torresen Marine (Yanmar dealers) have said that the newer multi-grade oils, such as Shell Rotella 15-40 (although I may have those numbers wrong, maybe 15-30?) are an upgrade in oil for these older diesels. I changed to the Shell Rotella in my boat with a Yanmar 2GM20F when I bought it 3 years ago and the engine runs fine. Check
www.marinedieseldirect.com for more info. Torresen also has a bulletin board there for Q&A's as well.