Based on CaptnWil recommending synthetic oil and his endorsement of Mobil Delvac 1, I spent some time speaking with engineers and industry experts -- everyone I spoke with said that Mobil Delvac 1 was probably the best marine diesel engine oil that money could buy. When I spoke with Mobil, they referred me to the following web site which gave the following test characteristics for Mobil Delvac 1 http://dallnd6.dal.mobil.com/gis/MobilPDS.nsf/938dd40f165c0309862566660070e8 7c/022b5cb5b179a4eb8525670b0059271d?OpenDocument Typical Characteristics Test Method Mobil Delvac 1 SHC SAE Grade - 5W-40 Viscosity cSt at 40 oC D445 87.9 Viscosity cSt at 100 oC D445 14.5 CCS Viscosity, Poise @ -25 oC D5293 31.7 MRV Viscosity, Poise/PA @ -30 oC D4684 94/<35 Viscosity Index D2270 172 Sulfated Ash,% D874 1.8 Total Base Number D2896 15.9 Pour Point, oC D97 <-54 Flash Point, oC D92 226 I am not sure what all of the these numbers mean -- however, 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. I haven't changed from using a straight 40 weight oil recommended by Bob Smith -- but I would appreciate hearing comments by those on the list that are more expert in this area. Thanks Al Palm Coast, Florida Pacific Trawler 37