Another 'you need zinc' thread? Oh yeah!!!
First, the oil of today has THE SAME AMOUNT OF ZINC FROM THE 50s!!!!!!!!! THE SAME AMOUNT.
The reason zinc was excessively added to oil in the 70's was because its a cheap anti-oxidizing agent. It was never added to street cars for protection of flat tappet cams.
Flat tappet motors are still being made today (they call them inverted bucket) and are usually in lower performance engines because of cost and modern oil still provide sufficient protection with the current API standard.
The ONLY VALID reason for adding zinc is because of a high performance cam with double valve springs and the like. Even that is questionable because if you have such a high performance cam you're gonna be using roller lifters, not flat tappets. So if you want a whumpa whumpa engine and still run flat tappets, they yeah, you're gonna need something like ZDDP to make it last... but zinc over 1200ppm is a waste and WILL start to etch the metal making more friction etc.
So unless you know you have a high lift cam and very stiff (outside factory spec) valve springs adding ZDDP, using synthetic oil or pouring unicorn tears into the oil is not going to do anything but waste money.
And no, you don't need ZDDP at break in. Assembly lube is all you need.
Que all the race car experts that say otherwise (just remember, I never said anything about race cars... I EXPLIXITLY stated that high lift cams/high tension valve springs *might* need additives... but really if you're doing high lift the smart people will have switched to rollers).