Many gears can be affected by a phenomenon known as micropitting. This condition is seen when microscopic cracks form on gears and through time and stress result in microscopic pits. These pits grow larger and eventually break away. This can even be a primary failure mode for gears.
Zinc has been a key additive constituent in hydraulic oils, as well as the majority of oil types, for decades. It primarily is used as an anti-wear agent or as an antioxidant.