Marvel Mystery Oil, Bardahl, etc. are products that had a certain amount of effectiveness in certain conditions thirty years ago.
One of the most highy improved automotive related products today is your engine oil. The additive packages in modern motor oil are the TRUE "marvel."
That said, I keep a can of Marvel Mystery Oil in my shop all the time. I use it to lubricate my air tools. I make real sure that it doesn't get any closer than that to my cars.
My recommendation is to use the correct engine oil from your favorite producer. As far as the additives, the old spaghetti sauce commercial said it best; "it's in there."