Why keep beating a dead horse? Several of the oil companies manufacture oil specifically for our old flat tappet engines. Watch out with the new shell rotella oils, the CK-4 rated oils are limited to around 1000-1100ppm of zinc. Some of them are CK-4/SN rated and those are 800ppm.

I like amsoil but there's plenty of other companies in the game. Amsoil's break-in oil has 2265 zinc. Their Z-Rod oil made for our old cars is SL rated and 1440 zinc with an extra additive package or corrosion control during long-term storage. Their dominator racing oil lists over 1500 zinc and isn't even API rated at all. For the one oil change per year I do on my old cars, IMO it's not worth cheaping out and worrying about what I can get at walmart.

https://www.amsoil.com/techservicesbulletin/MotorOil/TSB%20MO-2007-08-08%20Flat%20Tappet.pdf