Think about it. Just how high would the zinc content have to be to result in such unreasonably high levels of copper?
It's a nine-fold increase! As one of my chemistry profs used to say when discussing Avogadro's Number, "That's a HELLUVA lot"!
How long have you owned the truck and how long have you been using "high-zinc content" oils? How much higher are the zinc levels in this oil you've been using with the higher zinc levels?
It's late, I'm tired, I don't feel up to the task of even beginning to derive the calculations necessary to "estimate" the amount of copper that would be leaching out of the oil coolers-much less the amount of zinc that would be coming out of the brass. For that matter, I don't see a reason for zinc in oil to be attacking the brass in your oil cooler to begin with.
While I managed elemental spectroscopy labs for quite a while-all but one of which did oil analysis (when Blackstone Labs is in the same town where you own a testing lab, why bother duplicating the effort they are already doing?), it's been 22 years since I did it daily.