STP used to consist of polyisobutylene and a bright base stock.

As for adding STP, there is nothing in my experience to be gained, and as for me, I don't want the ring belt deposits. If you're using a high quality motor oil (Delvac, Rotella T, Kendall Super DIII, etc.) there's no advantage to adding something else to the oil. In the oil biz, these additives are called "mouse milk".

As for zinc dithiophosphate, why would you want to add it? It's probably already in the oil, in the desirable proportion.