The Zinc motor oil story has almost become the stuff of legends. I have seen more people haggle over this issue (heatedly) on automotive forums than any other. It has been very well demonstrated that Zinc provides wear surface protection benefits at break-in. What folks seem to have forgotten is that extensive testing has been done on Zinc additives to motor oil, from the 1930's thru the 70's. People also seem to forget that Zinc is a phosphorous substance. It's right in the name, after all: ZDDP (Zinc Dialkyl Dithiophosphate)
Phosphates such as Zinc do indeed provide additional lubrication benefits in very small amounts. Phosphates, however, will actually increase engine wear significantly when added in larger amounts (concentrations above 1500 ppm.) Engine rebuilders recommend a concentration of 1200 ppm for break-in only. This is why I cringe when I read how posters on automotive forums enthusiastically discuss how they use Zinc oils like Rotella, and then add "a bottle or two" of Zinc additive to their engines...even long after break-in.
The principal benefit from Zinc is that it leaves lubricating film on wear surfaces. Consequently, startup will be the primary protection zone that Zinc comes into play. The oil itself takes over after that. Some motor oil companies may have done away with Zinc in their oil, but no one seems to understand that in those oils, Zinc has been done away with in favor of other lubricant enhancers, like Molybedenum and Boron...which don't actually cause the damage that phosphates like Zinc do. Molybedenum and Boron also leave a film on wear surfaces after shutoff.
Its fine to add Zinc to your oil after break-in if you feel the need. But understand that it's not a "more Zinc is always better" proposition, as many seem to think these days. Using Zinc in concentrations above 1500 ppm for extended periods will cause problems like pitting on wear surfaces and camshaft spalling. That's not conjecture. It's physics.
