Best advice is to avoid oil additives and stick with the additive package that comes with the oil. Since you (and the additive company) have no idea what the oil company package contains, you could just as easily add something that hurts performance as you could add something possibly beneficial. The chemists who formulate a modern oil like Mobil 1 are probably a little more qualified to decide what needs to be in the oil than you or I.
BTW, have you ever heard the expression "snake oil"? Most oil additives fit into that category, especially the ones like Slick 50 that promise all kinds of BS benefits (wonder if it regrows hair?). Put that stuff in the same category as the "Turbulator" fuel economy improver. Keep your money in your pocket, or if you just have to spend it go buy something useful like another chrome doodad.
Jerry