Why would you do that??
Now you need an oil change too. A whole quart? MMO is something you used to squirt into the oil fill to mix with the oil to allegedly clean up deposits on the cylinders and even bearings and valves and whatnot, BEFORE just about every oil came mixed with thinner oils and detergents for just this exact purpose, and before every car began recommending exactly which oil blend to use!
Since when does it even come in quarts?!?
Marvel Mystery Oil is a very thin oil that again, in limited amounts may help clean up mostly carbon deposits and also other long chain molecule films or whatever else that might cling and bond with the metals in your engine. It would do this by getting very hot and melting and burning these deposits away.
Most oils now do the same thing without ever compromising the protective properties of motor oil. You don’t even have leaded gasoline anymore. The days of carbon deposits on the whatever, rings, pistons, valves, what have you, are gone.
I notice you say it seems better after you added a quart of MMO. That might even make sense, but in the interim, you need an oil change back to your normal oil. I’m not even sure what my original suggestion was now. Just get an oil change, using the oil that the manufacturer recommends.