I hope I don't hurt any feelings here, but you asked if anyone knew Marvel Mystery Oil. You got answers from some who do, and some who don't.

I need to give you a short history about me so you'll understand why I'm giving my opinion on this subject. My Grandaddy started his own automotive machine shop in 1929. Each & every engine custom built for the application it was going to be used for (not production rebuilding). I grew up in his shop, and built engines there for over 15 years. We have always hated any additives, Except MMO! I have seen first hand what additives can do to (or for) an engine.
Grandaddy used MMO since it first came on the market, and it is the ONLY additive that he would ever recommend to a customer. MMO can be used in 2 ways for an engine.
1- In the crankcase. It does remove or prevent varnish or sludge deposits in the engine (which by the way are caused by parrafin based oils and by short trip driving {point a to point b without time to heat the engine up good}.

2- In the fuel system. MMO does not boost octane at all. But it does remove or prevent carbon buildup on the valves and in the combustion chamber. Which is the main cause of detonation (preignition). MMO also does NOT leave any kind of deposits on valves. If you had the opportunity to disassemble a 283 chevy out of a 2 ton wheat truck with 320,000 miles on it that has used MMO in fuel and crankcase since it was new, you would be a firm believer also.
So, now that all that has been said, to answer your question. Yes. If you are storing your car for long periods, or driving a lot of short trips, it will help to run a little bit of MMO, in the fuel and the oil.
And for those with lifter noise (mine doesn't have this problem), unless it is a mechanical problem, try it you might be surprised.