When I said oil of the day, I wasn't speaking about synthetic vs. conventional. What I was talking about was the ZDDP content. When our cars were designed, the oil of the day (conventional or synthetic) had a ZDDP content of 1000 PPM, give or take, and was a required part of the SAE grade of the day. The engines in our cars were designed around that amount of ZDDP content, so the curvature on the end of lifter, the amount of pressure the hydraulics of the lifter exerted, and even the cam lobe shape were designed assuming that there would be ZDDP in the amounts specified by the SAE grade requirements. Move ahead 20-25 years. Research/environmental assessment/investigation/etc. comes to the conclusion that ZDDP is a major contributor to premature catalytic convertor failure, so the SAE SH and later grades started to reduce the ZDDP content. Not really an issue, since newer cars were using roller hydraulic lifters, so the need for ZDDP was vastly reduced. Just fine for any roller hydraulic lifters equipped engine. Not so for cars that were designed with the assumption that a much higher level of ZDDP would be present (almost all of our cars, aside from the late M bodies that had hydraulic roller lifters). Unless modified, they still have lifter and cam designs that assumed ZDDP would be present in the 1000 PPM level. As I said before, modern off the shelf oil, either conventional or synthetic, doesn't have ZDDP at this level. I would like to find the high ZDDP level Amsoil synthetic locally, but haven't been able to do so as of yet. So I will go with Lucas Hot Rod oil, which does have the required ZDDP level, although it is a conventional oil.
