Bearings are what allow the gears to rotate efficiently. They are usually manufactured from bronze or brass, they usually in good shape into races (or grooves) from the gearbox housing. That is definitely why companies came up with interior filters. These filters can be found on many contemporary automated transmissions. https://rylandwlzl.ltfblog.com/33043138/new-step-by-step-map-for-filter-gearbox