MSDS available from STP says 1% to 5% Zinc compounds by volume. When diluted in a crankcase, I think that comes out to about .2% ZDDP average. 1993 spec for automotive oil was .12%, and flat cams were uncommon. Current API spec for automotive motor oil is .08%, and reports of flat cams are widespread. Up and coming API spec is .05%.
CJ-4 is .12%. Add the STP to it and ZDDP will increase to .32%, maybe half a bottle would be better. Use of GM's Engine Oil Supplement would be fine too, perhaps better, it is listed as <5% ZDDP by weight.
