I'm probably beating a dead horse as this topic been on here before. I'm sure most teh avid gearheads know about the need for ZDDP but its worth mentioning for those of us that are getting back into it. I found this pdf from Howards previosu thread that shows the steady decline in Zn/P. also found this below on a Ford blog. looks like STP might be a bit low if you factor in SN API, certainly better than nada...
(re)
It shows STP to have 1959ppm of zinc and 1814ppm of phosporous. So, that would be diluted to 391ppm zinc and 362ppm phosphorous when added to five quarts of oil, right?
Rotella 15-40 which seems to be accepted as having enough zinc to prevent flat tappet wear contains roughly 1200ppm zinc and 1100ppm of phosphorous.
Current passenger vehicle oils on the market now are SM spec and contain approx 870ppm of zinc and 800ppm of phos, therefore adding a bottle of STP should bring that level up to roughly 1261ppm zinc, and 1162 phos.
The conclusion I've arrived at here is that running with a bottle of STP added to SM spec oil is as effective as running 15-40 diesel oil in our effort to maintain the service life of flat tappet cam engines.
Interestingly, I've also read that adding STP to 10-30 will increase viscosity to about 15-40. So, adding STP to 5-20 should bring it up to 10-30 if you are looking to maintain that viscosity.