...It was assumed that the Zn-contents of the oils samples were entirely in the form of ZDDPs and that the oils contained no other additives with Zn. The data 442 S. Ferguson et al. / Physics Procedia 66 ( 2015 ) 439 – 444 provided by one manufacturer indicated that this assumption was correct (Tyger, 2013) and the authors of this report know of no common oil additives, other than ZDDPs, that contain Zn...
Not necessarily a valid assumption. There are other zinc chemistry, such as zinc naphthenates, complex zinc diamyldithiocarbamates, sulfurized olefin-zinc dithiocarbamates, and neutral zinc dinonylnaphthalene sulfonates.
As far as the phosphates, there are Multi-Functional's such as the phenol-isopropylated-phosphates and the Borated Phosphates (phosphorilated borates).
With the proper ratio of sulfurized olefin-zinc dithiocarbamate, and the phenol-isopropylated-phosphates, one can make an Anti-oxidant/Anti-Wear chemistry almost identical to ZDDP.