I have a sneaky feeling that Marvel Mystery Oil is nothing more than a high quality ATF lubricant. I always run a quart of ATF in my engines for about 500 miles before draining. If I remember correctly they have a lot of good additives in tranny fluid, which keeps all the seals soft and it has some great detergents that remove old gunky carbon deposits.
I don't know about you guys but I can get a quart of ATF for $2, compared to a little dinky bottle of Marvel Mystery Oil.