I've also read that zddp is more critical when running high lift cams. The higher the lift the higher the spring rate causing more force on the lifter to cam lobe...pressure. So by that I gather a stock 318ci 2 bbl engine doesn't need near as much zddp than that of a highly modified 440 with a high lift cam. Something else I've studied is that 1000 to 1200 zddp is plenty for a stockish engine. Personally I run 15w40 Shell Rotella T or Dello400 LE with a half bottle of Red Line lead additive. Dello400 LE has 1300 zddp. With the half bottle of Red Line it's up to approximately 1500 zddp.

BTW, 2 years ago I broke in a high lift cam of 0.518" lobe lift with 15w40 Shell Rotella T and a bottle of STP in the RED bottle. No problems.

They don't make the STP in the red bottle any more. It is/was high in zddp.