In my 91 Regal Custom 3.8, I heard that putting in a quart or something like that of Marvel Mystery Oil is good for it, my car only has 133K on it. I just want to make my car run for a long time and to prevent things from happening.

Actually, I've been adding MMO to my fuel for decades. (((>four ounces for every 10 gallons. <)))
It has also been recommended, by those considerably for mechanically talented than me, to add about 15 ounces to the oil,
about 300 miles prior to fully changing the oil. (((> and filter <)))