I don't have an opinion one way or another, but here is the quote on MMO that I think the thread starter is referencing...
(Become an advocate to have Marvel’s Mystery Oil gain FAA approval. When mixed with fuel, it burns almost like fuel thus no detonation problems associated and when mixed with fuel at a ratio of usually 4 oz. per ten gallons of fuel added, it lubricates the rubber parts within your fuel system to help keep them soft and supple because when your rubber parts inside the fuel system become cracked from the otherwise dry fuel, the system begins to fail and can even kill your engine, and of course fuel systems are expensive to overhaul. Further the Marvel’s will keep your valve guides lubricated; a weak point of the Lycoming engine especially. Additionally, MMO will soak, loosen, and remove carbon buildup inside your cylinders. Some people even put it in their oil 1⁄2-1 quart at oil change because when the engine is shut down, the oil falls off of the ferrous metal parts and they become exposed to the beginnings of corrosion. MMO will coat those parts and stay coated when even lessens the wear from cold starts before the oil can circulate. Did you know that about 80% of engine wear comes not from flying the engine, but from starting it? If you know your airplane well, you will feel a slight elevation in vibration. That is because the MMO is removing the carbon which in turn goes out the exhaust valves making them shake just a bit, but once the carbon is gone, that engine will never have run as smoothly and if you were to miss putting some in when you refuel, again if you know your bird well, you will feel the difference. MMO is owned by the Turtle Wax Corporation last I checked and they should be rallied to get MMO approved for use in aircraft. Heck, ask any Harley driver if he uses MMO, Most will say heck yes! MMO works best in air cooled engines, but I also use it in my ferocious ’97 hotrod Lincoln MK VIII fun car. I also use it in my riding mower’s engine as those like to carbon up easily that this prevents that from forming. I had the head off recently and there was no carbon inside and the piston looked brand new.)