A friend of mine called today to ask an arcane question that I can't answer, but I know someone here can. He manages a farm for a guy who has an aviation repair service. They often have to drain fuel out of helicopters that are being repaired. They want to recycle this fuel in the farm equipment, which means a diesel tractor and a '97 GMC diesel pickup. The aviation mechanics are saying to add one quart of 90 weight oil to every ten gallons of the fuel taken from the helicopters and it's good to go. My friend is a little doubtful about this money saving experiment, particularly with regard to the old diesel pickup. What say the experts?