----- Original Message ----- From: "Dr. Al Miller" <almiller@n-jcenter.com> > an engineer at > Mobil told me that Mobil Tri-Synthetic Motor Oils are very similar in test > results. He recommended Mobil Tri-Synthetic 15W-50 for my American Diesel > 6N140. He said that I could safely double the oil change interval suggested > by the engine manufacturer. > Hi Al, I find the advice given you by the Mobil Engineer to be suspect. While the Tri-Synthetic Mobil 1 may work just fine in a Lehman or American Diesel engine it is apparent from reading the spec sheet that this oil is not primarily intended for diesel service. Why is it apparent? This is a new formulation and it meets the latest API "S" specs but it only meets API CF which is several years old. Also, note that the proper name for this product line is "Mobil 1 Tri-Synthetic series". "Mobil 1" has always been an automotive oil not a "heavy duty" oil. Mobil's HD synthetic engine oil is called "Delvac 1". Further, unless he specifically addressed the single vs. multigrade issue for Lehmans and American Diesels one must assume that he hasn't considered those issues. BTW: While at the Poulsbo WM Trawler Fest I had a brief chat with a guy from Lugger about our recent questions around why Lehmans want single grade oil. He felt that it was probably related to piston ring clearances and valve guide clearances. That is, and engine with looser clearances would need a heavier oil to make up for that looseness. I'm not sure that I buy that argument but I pass it along as another data point. Suspiciously yours, Alex