I've been curious about something for a while. From everything I've read here & elsewhere, it seems that from the 1960's thru at least the late 1980's, folks had all sorts of problems with 10w40 motor oil. Most of the complaints focus around the "fact" that it broke down early, leaving behind excessive deposits & sludge. On the other hand, an oil of similar viscosity, 15w40 HDEO, is constantly praised for the performance it has consistently delivered. My question is this: Have any of you BITOGers ever owned a car (with a known oil change history) that called for a 10w40 oil, but one in which you used a 15w40 diesel oil instead? If so, what were your observations using a 15w40 diesel oil over the gas-engine rated 10w40?