I have heard that 1 qrt transmission fluid does work well to clean an engine gently. I have a 1989 Buick full size estate wagon that I use Marvel Mystery oil in the oil (1qrt with 4 qrts regular 10w30 motor oil) and I also use it in the gas with every fillup (8 oz for 16 gallons) What a difference it made in acceleration and smoothness of the engine (the car has only 35K original miles with the olds 307 engine) When I bought the car it was very sluggish. After 1 oil change and 2 tanks of gas it was a totally different car. It will actually get out of its own way now Don Jr.