Jeff
This is a good article on ZDDP.
ZDDP: When, Where, What, Why, How? - Engine Builder Magazine
The limits on ZDDP and in particular, Phosphorus are related to poisoning of the catalytic converter. I used to use Shell Rotella until they dropped the amount of ZDDP in it. Brad Penn and the Joe Gibbs oils are "racing" oils and have substantially more ZDDP in them. When Keith Craft did my 427 center oiler, he insisted on the Gibbs
oil to save the cam. I told him I was using Brad Penn and he said that was just as good.
Tim