I do not recommend adding additives to fully formulated engine oils. OEM-style flat-tappet valve trains only require around 1200 ppm of phosphorus (the anti-wear component of ZDDP). You're much better off getting an oil that already has the right amount of phosphorus. The latest API CK-4 heavy duty engine oils have enough ZDDP as well as detergents to keep your engine clean.
One more bump on this thread - CK-4/CJ-4 are diesel oil specifications - they get around the reduced ZDDP regs because such engines don't run a catalyst.
What I find most curious about this 'formulation' concept is that it appears that the additive manufacturers (Lubrizol, Rhein Chemie) appear to be doing all of the engineering whereas the oil companies are simply buying and blending in additives.
Lubrizol sells at least 7 different formulations of ZDDP:
Tri-Iso Tryline LLC whereas RC is selling less-costly substitutes based on ZINC-DI.
None of these additive packages appear to be directly available to consumers.
