Zinc

I have done a little more reading on the zinc and phosphate additives. STP does have some but it's mostly just thick oil that will increase viscosity. That could be good and it could be bad...depends on your engines needs and what you start with (engine oil viscosity).

I found that Redline Break-in Oil Additive is a better source of ZDDP. Because it is very concentrated, you only have to add a small amount to an oil change and that will bring your levels up safely.
Check out the chart at the bottom of this page:
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1600403