Ok, I've run Case(construction equipment) 15W40 engine oil in my truck for a few years, with no apparent problems. I've quite recently switched to John Deere 15W40 plus 50 in both my truck and my wife's car- mainly because I've started working at a john deere dealership and get leftovers for free. My question is: Is there a significant difference between oil designed for gasoline engines as opposed to oil designed for diesel engines? If so, what's the difference? Am I dooming my ford 300 I-6 and my wife's 3.1L V-6 to an early grave? A feller I worked with at case told me once that he thought the "ash content" was the main difference, and that running diesel oil in a gas engine isn't good. Obviously, I didn't put too much stock in that... anybody know for sure?