I use it...
right before doing oil changes on my moms Rav4 with 150k miles that she thrashes
(i got a half gallon for free too)
http://en.wikipedia.org/wiki/Marvel_Mystery_Oil
copied from another source:
Marvel Mystery oil is 30% varsol (charcoal lighter fluid) and 70% 30wt base oil some red dye and wintergreen fragrance. It is from the late 1920s.
Its a solvent type product. Some people find it beneficial for sticky valves or some sludge dispersal. If you are depending on them for either of these reasons, you have bigger problems.
It evaporates within hours in an engine. I neither provides any corrosion protection or anti-wear benefit. It will not harm anything in the recommended concentration.