Zinc Rich Oil for Flat Tappet Cams

Those most directly responsible for the reduction of ZDDP in motor oil are the vehicle manufacturers. They lobbied API and ILSAC to reduce the amounts of ZDDP in the newest oils certified by those organizations, because the OEMs are required by the EPA to have a catalytic convertor warranty of 8 years or 80,000 miles. As we know, the virtual elimination of flat tappet cams has drastically reduced the amount of ZDDP needed as an anti-wear additive. Therefore the OEMs don't want to be using something they see as not only unnecessary, but harmful to their bottom line.
So, it was caused by the government through their 8/80 mandate, but really pushed by the OEMs to protect their bottom line. That's it in a nutshell.

I don't like to use additives whenever possible. Chemical and petroleum engineers work very hard to come up with balanced formulas which can be upset, and made considerably less effective by the introduction of an additive solution. My oil of choice for my stock, never apart, original 340 is Rotella T6 5W-40. Synthetic, low ash, 1200-1300 ppm of ZDDP, and only a 40 weight on the hot end of the scale. Recommended by Shell for gasoline vehicles. Available at Wally World for $20/gallon. And they're the ones who send my pension check every month.