OK, had to jump in here on this Synthetic Oil subject. The following is from REAL LIFE experience using Synthetic Oil in not only 5 power boats and 3 sail boats, but also in our company that runs 125 light duty delivery trucks and about 20 cars. Another division (long haul over the road) operates 28 tractors, and 35 refrigerated 53' trailers. ALL of them (and my boats) use Mobil ONE DELVAC SYNTHETIC oil. (NOT the same as Mobil one in the auto stores. that is only for gas engines) But, you can use Mobil ONE DELVAC in gas engines. General get 400-500 thousand miles out of the light duty 4 cyl diesel engines (15,000 mile oil changes) in the delivery trucks and One Million Miles + out of the big Tractors (25,000 mile oil changes). At tare down, or rebuild the engines are as clean inside as they were new. NO built up of sludge at all. Reefer engines generally run for 25,000 hours before rebuild. It is very true that the Synthetic Oil stays very clean for many more miles / hours then mineral oil. Now to boats. Boat engines are under "LOAD" all the time pushing a boat thru the water, the demands on a marine application is even more important to run Synthetic Oil. On one of my boats, with two 500 HP Yanmars, the "by pass" filter spun off and dumped 5 gal of oil in the bilge in seconds. Lucky I was at the helm and saw the oil pressure gauge bouncing around. I do not know EXACLTY how many min the engine rain with damaging low oil pressure or lack of need oil before I shut it down. What I do know, is that I am sure that if I had had mineral oil in the engine instead of Synthetic, the out come could have been much worse. Read other post about all the QUALITIES of Synthetic over Mineral Oil. "Film Strength" and ablity to withstand very high temps and still not breakdown its quality of what oil needs to do, is why I think ......... I never had a problem with THAT engine that ran for some time with very low oil pressure and lack of oil. Also, Synthetic's help to not burn oil on long hour runs on a boat. Running from NJ to FL outside and inside, I have never added a quart to the whole trip on the different power boats that I have owned. Cat's / Cummings / J Deer. Of course, these were all NEW boats and from the very start I used Mobil One Delvac in them. Oil was clear and clean at the start of the trip and 1,200 miles later, still clear and clean. NO BLACK OIL. So, we all have experiences with what we like to use in boating. Synthetic's may not be for everyone for sure. I would only say that Synthetic Oi willl give you a great peace of mind (at least to me) that my engines are getting the VERY BEST possible chance to live a long and healthy life, with an extra margin of safty if needed. Oh by the way, a friend of mine is using Mobil Delvac One Synthetic oil in a 4 cyl Yanmar (160 HP) since new and now has 9,000 hours on it and running like a top. Still has clean oil in the engine from oil change to oil change. I have NO interest in Mobil at all. There are other brands of FULL Synthetic OIL that I am sure are just as good. I just happen to use Mobil brand Synthetic Diesel oil. Bob Wilkins