Plagiarized from a boat forum.
Marvel Mystery Oil is a light napthenic oil based solvent and lubricant and has been around since the early 1920s.It was owned by the Marvel oil company up until 5 years ago when turtle wax purchased it. People have alot of disagreements about whats in MMO, how it should be used and its safety. The entire formula is still proprietary and a "mystery",but its basic ingredients are napthenic oil, oil of wintergreen and mineral spirits. Some folks claim it also has some phosphorous in it. No,it is not kerosene and ATF. As for its uses,it is a good product to use to break loose an old engine that hasnt been run for many years.It is useful as an engine flush,although some disagree about whether it should be left in the engine and driven or used only at idle.It is also useful in diesel engines and 2 cycle engines such as boat motors,lawnmowers,etc.to reduce smoking and deposits.As for use in modern cars with fuel injection,electric fuel pumps,complex emissions systems etc.,alot of people say it probably should not be used in these vehicles.Although some claim it is an effective fuel injector cleaner and mileage improver when used in gasoline.You have to use your own judgement.