I had heard, back in the seventies, that Mobil was the first refinery to have all three grades pass the BMW test which consisted of taking a new engine apart, weighing the valves and reassembling it. Then they ran it for 50,000 miles (simulated I suspect), taking it apart and weighing the valves again. Whatever tolerences were allowed, Mobil passed with even it's lowest grade. I have not done any research on this but I am wondering if anyone has heard of this "BMW Test"?
