It's been a little while since I've posted here, but I have been following the threads, especially this one. I located a link which I will share with the group, and it concerns Amsoil and the use of ZDDP in engine oils. I found it to be an interesting read, and had lots of useful information, especially on engines using flat tappet cams like the Big Block GMC V-6's. I do hope you will follow the link, since the info contained in the article should answer many of your questions involving the use of ZDDP in engines, especially older, and high performance engines.
The Debate Over Reduced ZDDP and Wear Protection - Amsoil
https://www.amsoil.com/newsstand/classic-and-vintage/...