Over the Counter Zinc Additive
The mention of STP got me curious and I found 2 oil analyses for the additives in STP.
The older one (2005) showed zinc at just under 2000 PPM in a 15 oz bottle opf the 4 cylinder product; that would raise ZDDP about 200 ppm if the whole bottle was added to 5 quarts of oil.
STP Red 4- cyl Oil Treatment | Virgin Oil Analysis - PCMO/HDEO | Bob Is The Oil Guy
The newer analysis (2012; the blue bottle stuff I presume) showed that the levels had dropped to <600 pmm in a 15 oz bottle, and that would leave the ZDDP levels essentially unchanged if the whole bottle was added to 5 qts of oil.
STP High Mileage Oil Treatment VOA | Virgin Oil Analysis - PCMO/HDEO | Bob Is The Oil Guy
So I have to wonder if the ZDDP has been seriously reduced in STP.... so as not to violate the new oil content mandates.... and if the 2nd link is accurate.