Re: Calling LD, re: an oil question.
Chris, ash content has very little to do with "by-products". I'll try to give you the real answer in layman's terms....<br /><br />Sulfated ash content is directly related to the oil's total base number (TBN), which measures an oil's alkalinity and ability to neutralize acid.<br /><br />As the total base number increases, the sulfated ash content also increases. So I suppose you could say that in general, high-ash oils will have a more robust additive package, and low-ash oils will have a less robust additive package. <br /><br />In diesels, higher levels of ash and a more robust TBN is necessary to offset the harmful effects of acids formed during the burning of high sulphur fuels. And to answer your question, this is exactly why diesels use higher ash containing oil.<br /><br />The problem is that ash is not necessarily a desirable component, especially if you don't need it to offset the effects of the combustion of sulfur-containing diesel fuel. High ash oil can leave more sludge and deposits in the engine, particularily a gasoline engine.<br /><br />Of course using an oil with a ash content that's too low in a diesel could lead to engine failure, generally valve failure.<br /><br />Therefore, I would suggest that if you're going to use a diesel oil in a gasoline engine that you find a diesel oil with the lowest ash content possible. Or better yet, use a gasoline rated oil rated in your gasoline engine, and a diesel rated oil rated in your diesel engine. An oil carrying both ratings would work fine, but may not be the best for both engines. You may want to double check if the 15W-40 is a recommended viscosity for your gasoline Mercruiser. Also know that multi-viscosity oils usually have viscosity improvers in them which break down long before a straight-grade oil. Personally I'd follow the manufacturer's recommendations, probably using a straight-grade marine diesel oil in the diesel engine, and a straight-grade marine gasoline oil in the gas engine.
