I've been using Marvel Mystery Oil (MMO) since the early 1960's (and I think it's been around since the 1920's), usually as a penetrating oil and/or sludge dissolver, but sometimes just as a lubricant, although I believe that the main component (that which carries the solvents and actual oil and makes up the majority of the ingredients - probably Acetone, but I don't know) evaporates over time (but then, so does WD-40 and that stuff isn't even much of a lubricant, either). If you do a search on MMO you'll quickly find that the only people who know what's in it are the chemical engineers at Turtle Wax (the owner of MMO) and they're not telling.....Everyone else is just guessing, although I found a pretty extensive post from the RX7 club and another one from an off-roading group. Everybody who's used the stuff, in cars, boats and airplanes, thinks it works great at loosening things, especially pistons and stuck valves.
I've found it pretty effective for dissolving sludge and the coke that gets deposited on valve's and their stems, and have used it more than once as Wolfgang described: pour 4-5 ounces into a frozen cylinder (usually a vertical cylinder), go away for a couple of days and then try turning it over. 1-2 doses usually works. I haven't filled the cylinder, but I suppose that would work as the stuff will leak out as the rings free up.
HOWEVER! Since the VW engine is horizontal, I would tend to use a Seafoam Fogger instead of MMO because it would tend to get penetrating oil up to the top of the cylinder/rings and do a better job. I would Fog it, come back in a day or so and fog it again, wait a day or so and then try turning it over. Or, of the engine is out of the car, stand it on one side so the other cylinders are straight up and hit it with MMO, wait an hour or so and flip it and hit the other two cylinders with MMO.
For a pure penetrating oil to loosen rusted fasteners, I use a 50/50 mixture of automatic transmission fluid (pretty much any kind but I had a bunch of Ford left over from my Pickup Truck) and Acetone. Works like a miracle if you're patient.
