We can all guess at the reasons why ZDDP levels in motor oil have been drastically reduced by the refiners but
none of us have any inside knowledge or verifiable proof of that reasoning. Stating such reasons as fact is just self-aggrandizement, as is
@Jerry Bransford's blanket statement that ZDDP "is just not needed for stock engines with stock cams" without any supporting evidence. Moreover, Jerry's generalization is misleading because unlike engines with roller lifters, engines with flat tappet cams and lifters do in fact benefit from small concentrations of the zinc/phosphorous additive.
To me, the most logical reason (if one needs but one reason)
appears to be that ZDDP is hard on catalytic converters rather than it being unnecessary. The potential liability stemming from the inclusion of chemicals with potential to harm expensive emissions parts would
seem to be of more significance to a refiner than the benefits of that additive, but that is conjecture. Further, what I am finding on the Internet are references to Federal regulations requiring that ZDDP not be included in formulations marketed for gasoline engines using catalytic converters but allowing continued use of the additive in motor oils specified for diesel engines and racing, which suggests that the "real" reason for the elimination of ZDDP from motor oil rated for gasoline engines is the governmental interest in reducing exhaust emissions.
In any event, if one wants a conventional motor oil with ZDDP one can simply use Shell Rotella T4 which is available in 10W-30, the viscosity recommended for Jeep TJ engines. However, one should be aware that it is rated by the government for diesel engines rather than gasoline engines.
https://rotella.shell.com/en_us/pro...526a3e9f8e7e4a73481fb83be9cd3a4/t4-10w-30.pdf
I used to use Shell Rotella motor oil in my CJ-7's 258 c.i. I-6 engine, but that engine has a different design than the 4.0 liter I-6 motor used in the later TJ/LJ jeeps. For my 2006 LJ I use Valvoline Maxlife synthetic blend 10W-30.