After doing some research on the internet I found that the only product that has zinc in it made buy STP is STP Engine
Treatment With XEP2. That cost you $17.95. The $3 stuff which is STP Oil additive dose not state or to be found to have zinc. I believe the STP Oil additve falls under the PTEF catagory.
I have looked on the STP website and can't seem to find that they even have the engine treatment.
http://www.stp.com/index.html
But I did find that the STP Engine Treatment with XZEP2 have got themselves in trouble.