Be very careful with Marvel Mystery Oil. It is a viscosity enhancer; i.e., it makes your oil thicker, and by so doing, it supposedly "masks" noises and such. What you do when you add it to 10W30 is to increase that oil to 20W50. So what have you gained?
But it is not a panacea for anything that is truly incorrect.
If you do want to use a low viscosity oil, then use a synthetic, since they will not "break down" in film strength, oil shear, or any of the other "bad" things that petroleum oils do. Notice that NASCAR, Formula One, and CART use synthetics in their racing engines, but then they are revving at maximum RPMs most of the time, and they cannot afford to have oil failures. I doubt that your engines are subjected to such loads.
So now that you are armed with some more facts, you should be able to make an informed decision which is right for you.....Andras