Basically, fuel, petrol (gasoline) is to lubrication what Idi Amin was to humanitarianism. The function of oil is to maintain a tough, thin, slippery film between the critical metal interfaces in your engine - on the bores, in the bearings, the cam lobes and valve guides, the timing chain - all the expensive stuff that moves really, really fast, with great precision. (...) Metal-on-metal contact is the enemy of longevity, and oil is the cure. Fuel in the oil degrades the effectiveness of the oil.