Original version Marvel Mystery Oil observation: 1999 Chevy Astro van V6 has had low oil pressure for all the 8 years I've had it, 40 psi on cold startup idle, 20 psi idle after it gets warm. Has 193K miles now, developed a ticking sound like a valve problem. Added a qt. of original MMO, it shut up the ticking immediately. 2 weeks later the warmed up oil pressure fell off to zero at low idle.
Tiny bit of gas pedal gave it 15-20 psi. Got the oil changed last week then with the 5W30 it's always gotten, no difference in psi at all. Have added 1.5 pints motor honey (STP Oil Treatment) to thicken things up, got it to stop going to zero. You see what this observation means................
(A): Original Marvel Mystery Oil should never be put in an old engine with a questionable maintenance (oil changes) history. It will dissolve the sludge that may be all that's holding the engine together.
(B): It might take the MMO a good while to clean things up, so immediate results aren't all it will do, patience may be warranted.
(C): Never leave MMO in an engine longer than it takes to do what you wanted it to do.
(Bonus observation): Super thick motor honey will increase oil pressure in a worn engine.