STP Oil Treatment or similar products such as Valvoline SynPower Oil Treatment, Maxlife Oil Treatment etc. should not cause any problem for your car. In fact, the 2 major ingredients in these oil additives are very well-known anti-wear and viscosity improver additives. The anti-wear additive is ZDDP (zinc dialkyl dithiophosphate) and the viscosity improver stuff is Olefin Paraffin.

Many oils have a low concentration of ZDDP to provide anti-wear protection, but the above-mentioned oil treatments have higher ZDDP doses. ZDDP is not only an excellent anti-wear additive, but also a very effective oxidation inhibitor. Olefin Paraffin helps to increase the oil viscosity index, which gives these oil treatment a thick consistency.

ZDDP is not crappy stuff like the disguised chlorinated paraffin molecules, or PTFE stuff used in other snake oil additives such as Duralube, Prolong, Zmax, Slick 50 etc. Under high temperature and stress, chloride ions will be created from chlorinated paraffin. As you know, chlorine is a halogen element (which means "salt-generating" by reacting with metals), so its chloride ions are extremely reactive and corrosive, especially in contact with engine metal.

One pitfall of ZDDP is that it is somewhat corrosive to silver alloy crankshaft bearings. However, I think all modern cars do not use silver alloy at all.

Also, I have had good experience with Valvoline Maxlife stuff, including Maxlife engine oil and Maxlife ATF Dexron III/Mercon.
They work like a charm, by dramatically reducing oil leaks and smooth out auto transmission shifts.

Eric