So let me get this straight...an older engine with a flat tappet cam that is waaay broken in still needs the zinc even if I'm using a full synthetic oil?
If it's a stock cam and springs no any SM rated oil is fine. The difference between the oil tht was current when your car was new and the current limits is 1200 to 800. Every article I have seen that was written by an actual oil company engineer have all stated that the current levels in SM rated oil are adequate for stock flat tappet cams and lifters.