STP is a "Viscosity Index Improver". Translation: It will make your oil thicker.
How thick? Nobody can predict this, as we don't know the current viscosity of your oil. All I can say is that I would personally avoid this stuff, as you could wind up with oil that is too thick to flow properly, particularly if temperatures drop. And, if oil doesn't flow properly, you could wind up with lubrication problems, and that could spell an early demise for your engine.
As to a fuel system cleaner, if your engine is in need of this, the only two brands that I would recommend are Seafoam and Techron (by Chevron). The other fuel system cleaners don't do much of anything except lighten your wallet, IMHO.