Alex makes some good points. However, my opinion is that the cost increase for synthetic oil is not really measurable in the overall cost of boat ownership. I pay $100 for 4 gallons of Mobil Delvac 1, which I assume to be about $50 - $60 more than fossil oils. I change the oil once, maybe twice, a year so the extra $100 or so that I pay for synthetics just doesn't amount to much. But then again, Alex said it was my money. Why I've chosen synthetic is simply because every independent report I've ever read places synthetics well above all fossil oils. I consider it good and inexpensive insurance for one of the most critical and expensive systems on boat. BTW, I don't believe synthetics allow you to extend the manufacture's oil change interval, but they do allow you to use those intervals with confidence. Using fossil oils I'm inclined to change my oil more often. I don't know why, but assume it's a carry over from the great job Jiffy Lube did in marketing the 3,000 mile oil change for our automobiles when the manufacture's recommended oil change intervals are much greater. Check out the following link and you will see that Mobil Delvac 1 is approved, at least for some applcations, for Volvo, DD, Cummins and Mack. http://www.exxonmobil.com/USA-English/Marine/PDS/GLXXENMRNEMMobilDelvac1_5W40 .asp The usual disclaimers apply, just a satisified Delvac 1 user. Gil Confidential Monk 36