Originally Posted By: Benzadmiral
My 1975 Ford Maverick at first got Quaker State, though I'm not sure of the weight -- 10W-30 or -40, probably -- and either Fram or Purolator filters. At one point I tried STP's then-new oil, which was marketed as being good for 15K miles, though I still changed it at 3-4K. Toward the end of my time with that car, I was using Castrol GTX, again either 10W-30 or -40.
At various times I added Marvel Mystery Oil and Bardahl Top End Oil to the crankcase. Nothing terrible ever happened, except that my wallet was a little flatter.
I'd imagine that even with that guarantee, a LOT of people changed that STP 10W-50 oil out at 3-5k. If it had enough detergents (IF) to get close to 15k, it probably did a pretty good job when changed at 3-5k.....