Zinc has qualities as an extreme pressure, anti- wear additive. Zinc compounds react with metal to prevent scuffing and wear. From what I have read, a level of about 0.12% is enough to protect most engines under normal use.

In the Pontiac OHC, the contact between the cam and the follower is constant metal-to-metal. It's not like a typical OHV application. The follower has a constant contact zone against the cam because it does not spin. The cam and follower have "sliding" contact. Zinc was part of the design criteria at the time the OHC was developed. Most of the OHC designs today use "buckets" that drive the valve directly off the cam. The buckets are designed to spin based on the cam profile. If an OHC design uses a follower, it's a roller follower.

As a comparison: In the OHV, the follower (lifter) is constantly spinning, reducing the metal-on-metal sliding contact to almost nothing once properly broken in. The contact location is constantly changing on both the lifter base and the cam. As long as the pressure applied through the lifter onto the cam doesn't cause "slipping" (as you would have with a new cam and heavy valve springs) the surfaces will smooth each other out and will roll rather than slip. But there is still a very slight amount of slip that is required to get the lifter to rotate. The small amount of Zinc used during break in embeds into the mating surfaces. Zinc is then needed to assist the oil in preventing that small slippage from turning into a wiped out cam.

Virtually all synthetic oils have no Zinc, or too little Zinc for the OHC. Plus, bearing clearances and designed-in clearances for the cam oiling system were developed with non-synthetic oils in mind. Synthetic oils rely on much smaller gaps to accomplish their wear prevention. Lash adjusters were designed with controlled gaps and metering holes that don't cope with synthetic oil, either.