Well, there were a number of things that Mercedes did with this car that were marginal - the cooling system being ine of them. Oil is supposed to (from what I understand) have more BTUs than diesel. It may also burn at a higher temperature than diesel whether it has more BTUs or not. If the cooling system is marginal, it would show up. When I started the thread, I wasn't sure. Now that I've done some more work on the car, I know for sure that there is more heat put into the engine when running on oil. The only thing I don't know is "why."