Those concentrations are above the levels ever factory specified and well above the needs of a standard slant six.
See
Grant "Oils for Classic British Cars" in a couple places including the motor oil history.
Also Widman's
overview of motor oil properties related to flat tappet engines.
Both cite and quote primary sources relating to negative aspects of long term use of zddp above 1500, particularly these two:
“Cam and Lifter Wear as Affected by Engine Oil ZDP Concentration and Type”
Loren G. Pless, John J. Rodgers, Fuel & Lubricants Dept., Research Labs, General Motors Corp.
SAE Report 770087, 1977
“How Much ZDP is Enough?” R.M. Olree, (GM Powertrain),M.L. McMillan (GM R&D)
SAE Technical Paper Series 2004-01-2986, October 2004
However also note that even as far back as the 1950s, some negative effects of concentrations over 1000 ppm were observed. So its all a balance. If one has an engine with high spring loads, the additive package and oil properties are going to have different priorities than a standard or even a factory high performance engine.
As far as Rotella T4 goes, I would check the recomendation date and compare the T4 specs and VOA from then to currently available product. There were major changes in the additive packages a few years ago.
