This is 1st hand experience. My Toyota T100 5vzfe truck has 282,XXX on it and I have fed it Amsoil HDD 5w30 since I bought it w/ 179,xxx on it. I drive to work a particular way ever day and there is a hill and it will kick down a gear if I don't go around 75mph and it drops to 55mph when topping the hill. The idle was like a cammed engine when in gear. I always will get a pending code P0171, especially in cool weather.
I had bought a quart of Kreen motor tonic some years ago and never used it. I ran across it while cleaning my garage a few weeks ago and decided to try it. I put 1oz in each cylinder and let it soak about 30 hours, I put 16oz in gas and remainder in fresh oil. When i installed the plugs and started it there were no strange noises, but it smoked for about 2 miles. The change was instant. I drove it passed where i work and it did not gear down and I could tell it had more power. The pending P0171 has not returned and I haven't reset the ECU. The rough idle is gone. I can go up that hill and maintain 55mph the whole way without gearing down.
The Kreen in gas had not time to work, so I give all credit to piston soak. The missing P0171 I guess could be attributed to the PCV getting cleaned by the Kreen, not sure. My oil did not change color as I've read by other posters, so I guess my engine innards was clean (it should be)
So, I think , no matter how clean your engine is, high mileage engines can get caked, gummed up rings.
I've never been so impressed with a product like this. It worked so well and so quick.
I ordered 4 more quarts and I'm going to do two more piston soaks, then do it once a year.