Because the EPA wants a minimum 100,000-mile lifespan from catalytic converters, they need a fighting chance at survival. Zinc in the oil undermines that survival. That's why both the automakers and Washington decided zinc had to be eliminated from engine oil.
Without zinc in the oil, wear for vintage engines with flat tappet cams increases exponentially. In fact, it is alarming how quickly it happens and how much damage it does. Zinc is crucial not just for cold start-up, but extreme conditions that make heavy-duty engine oils necessary for reliable operation. There's no magic in heavy-duty engine oil, just higher levels of ZDDP to help reduce wear.
The first thing you want to know about engine oil is if it has an "SM" rating, which indicates greatly reduced or zero zinc levels, which makes it harmful to your classic Mustang's engine. Do not use engine oil with the "SM" rating. Or, if you're going to use engine oil with the "SM" rating, use a zinc additive that will maintain proper ZDDP levels. Front and center for this issue is California, which wants all zinc eliminated from engine oil. Regardless of what government and industry mandate for engine oil, it is up to you to ensure sufficient zinc levels are maintained when you change or add oil.
ZDDP is crucial to engine wear and break-in issues because so many things need to happen when you fire an engine for the first time. When you fire a vintage engine with flat tappets for the first time and run it at 2,500 rpm for 30 minutes, you are work-hardening the cam lobes to ensure long life. Cam lobes not only move the lifters, pushrods, and valves, they also spin the lifters in their bores for proper function. The lifter and lobe must have sufficient traction for spinning to happen. This is why you don't want to run synthetic oil or friction-reducing additive during break-in. ZDDP helps lifter/lobe traction. It also works into your engine's hardest working parts.