I don't like the idea of adding anything to engine oil as there is a chance that the chemical makeup of the additive package in the oil can be compromised. Use an oil that has the necessary amount of ZDDP in it. Companies such as Valvoline, Amsoil and Brad Penn make oils with added ZDDP for use in flat tappet engines or you can use and HDEO oil such as Shell Rotella T or Chevron Delo 400. I've been using Chevron Delo 400 15W-40 in my Studebaker engines and in friends Chevy and Ford engines for well over 30 years with great results.
Zddp additives are used for break in, if you want higher zinc oil buy it formulated with high zinc already in it.
FYI most people think they need extra zinc when mostly they don't, even flat tappit engines don't need it when running stock valve spring pressure. Remember there are modern and organic anti wear additives in the oil that do their part too.
An off the shelf HDEO will work for almost all applications as I have half a dozen older engines/trucks and they all get 15w-40.
The only disclaimer is stiffer than stock valve springs/racing applications.
My brother has an older Porsche he tracks and uses Brad Penn, personally I don't think he needs it but it makes him feel better.