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Nostalgic

Well, I've actually tried what you're describing - kind of. I am/was a dealer and since closing automotive repair, found myself with a lot of lines that are just sitting on the shelf. I have the high zinc Z-rod stuff, and added a gallon or so to my oil change. It did nothing to the numbers for zinc since it's still diluted by 10 gallons of rotella. Maybe a point here or there, but no meaningful additive change outside of normal batch variances. The additive pack for a gallon of gear oil DOES show up on a UOA. Gear oil doesn't really change the viscosity much overall, but it's additive pack and film protection DO.

Ironic, the GL4 I was using became hard to find and expensive, so I quit adding a gallon. Not that I'm against it - in fact UOA reports support Rawze's theory and opinion on its use.

I also experimented with zinc additive. Of course even dumping 6-8 bottles in didn't sway zinc's ppm on a used oil sample.