~65% Petroleum Distillates (Mineral oil)
~30% Stoddard Solvent (Paint Thinner)
~1% Tricresyl Phosphate (an organophophate, used as a plasticiser and solvent)
~1% Ortho Dichlorobenzene
<1% Para Dichlorobenzene
Personally, I think it smells great, but I don't think it's a magic elixir for engines. It's probably pretty safe for freeing up lifters if you don't add too much to the oil. A snort or three in the gas is fine, too, though I probably wouldn't use it on modern stuff.
I have an old Jag 6 cyl that hasn't run in 5 years but will again soon when a fuel tank is sourced. I took the intake manifold off and cleaned out accumulated oil vapor gook. Was thinking about filling the cylinders with Marvel to soak the rings.
Don't forget....the presence of the chlorinated stuff . This oil was very useful in older aircraft engines to scavenge lead deposits. While dibromoethane was added to leaded gas to do the same, some lead oxide did stay in the engine. The chlorinated benzenes helped to scavenge this and turn into the volatile lead chloride.
Today, I think we use it almost entirely for the solvent properties. It will dissolve gunk.
I had a 99 Durango with a 360 that would knock on 85 octane regular or 87 octane mid grade in the summer. Forget turning the AC on too. 91 octane premium would be ok so I guessed it was deposit on the pistons related. A friend suggested MMO. It took about 3 tankfuls and I could run it all summer with the AC on on 85 Octane regular. All my vehicles get a splash with every fill up now... just because I can and want to.
It's handy stuff. I used it in the gas of my last Jeep religiously, it ran better and got better mpg too my guess working as an UCL. I've used it a few times as a crankcase cleaner, it is pretty mild in that regard but it can help. Had a couple vehicles that I got caught with summer oil in the crank when it turned cold, some MMO made a dramatic difference in cold starting until I was able to get it changed out.
Works really well as a gun cleaner too. Crud just melts off with it. I have used it as a gun lube as well, although it will dry out a bit faster than many gun lubes.
Then there's OPE....
Like I said it's handy, and not expensive either.