One other thing concerning zinc -as I understand it -and I am by no means an expert in oil chemistry.
There are two types.
One type combines with phosphorous to make up ZDDP or ZDTP. This mainly provides start up protection by attaching to hot engine parts.
The other type acts as an extreme pressure additive.
We’ve likely all read more about ZDDP than we care to know, yet understanding the basic chemistry may not be obvious to all so will attempt here to provide a simplified overview of this class of molecules and how they work. To amplify a bit on Comnoz comments above, I too am no expert on motor oil or ZDDP (zinc dithiodialkylphosphate), but do have a working grasp of the chemistry which may help us better understand some of the results we see.
To begin, the class of molecules known as ZDDP function as antioxidants, corrosion inhibitors and anti-wear agents. As stated by others, ZDDP is not a molecule, but rather a class of molecules. All ZDDP molecules share the common structure shown below (enclosed in red brackets), where S is sulfur and refers to the dithio part of the name, P is phosphorus which is associated with the phosphate part of the name and R and R’ are hydrocarbon groups or the dialkyl part of the name. These alkyl groups are the components that distinguish one ZDDP molecule from another.
The R and R’ groups or so-called substituents are hydrocarbons and can be long or short, straight chain, branched chain or cyclic structures, and can be attached to the parent ZDDP group by their ends or somewhere along the chain. All these variants in substituents give rise to the myriad ZDDP molecules available to the oil formulators. Note that Zn and P occur in a ratio of 1/2, and considering their molecular weight, should always occur at a weight ratio of ~ 1.0/0.95. Thus when you are reading oil specifications, if the zinc content is 1000 ppm the P content should be 950 ppm if both atoms are from ZDDP. If this is not the case, there is additional P in some other form and for some other reason (antioxidant, corrosion inhibition, etc).
So, why would we care about all these possible variants of ZDDP? Because the R/R’ substituent relates to the stability of the molecule, and the molecule has to degrade to function as an anti-wear agent. More specifically, ZDDP functions by breaking down (thermally or mechanically), losing the hydrocarbon substituents, and forming a zinc polyphosphate glass on metal surfaces. It is this zinc polyphosphate layer that provides boundary lubrication, i.e., lubrication in the absence of a coherent oil film, via the hard, durable, low coefficient of friction layer formed.
By selecting a specific ZDDP molecule or variety of molecules an oil formulator can tailor the lubricant to provide the desired performance. For example, if a formulator were designing a break-in oil, they would select a ZDDP molecule that had limited stability so that upon initial engine start-up ZDDP would degrade quickly to provide the zinc polyphosphate layer ASAP after firing. Such a formulation would likely also contain some ZDDP of intermediate and longer term stability to provide a continuum of protection. I suspect for a high mileage motor oil where immediate protection is not the primary need, the ZDDP selected would be a very stable molecule that would degrade slowly to provide protection over a long period of time.
The foregoing should be a reminder that although some given oil has a desirable level of ZDDP in it, we have no idea which one of the dozens of ZDDP molecules or range of molecules it contains and perhaps not be surprised that all oils with the same ZDDP level are not created equally and don’t perform equally. Furthermore, some other additive, e.g., a moly friction modifier may provide some synergistic effect with a specific ZDDP, so very possible that ZDDP is not the whole story.
There is also the possibility that a particularly desirable ZDDP molecule might have a steep cost/benefit ratio relative to other molecules, hence the formulator has to balance the formulation cost vs performance vs the marketer’s desired selling price.
The link below to a Joe Gibbs oil primer hopefully reinforces the key points made above.
https://www.drivenracingoil.com/docs/dro/RacingOil101.pdf
Here’s what Amsoil has to say about ZDDP and their recommendation of their two Z-ROD oil products for flat tappet cams.
https://www.amsoil.com/newsstand/classic-and-vintage/articles/reduced-zddp-and-wear-protection/
Possibly one of these Amsoil offerings may make it into Comnoz test cycle. However, I can also see that this could be an open ended test protocol by the time we are all satisfied with all the oils evaluated. How about GM’s EOS as an additive?