Contaminants can enter your gearbox oil via vents or even the fill tube during checking/filling. More often than not, nevertheless, contaminants are the results of standard wear. The gearbox filter shields versus these contaminants which assists make improvements to gearbox existence. Which is why companies came up with internal filters. https://filter-gearbox20852.ziblogs.com/34205885/filter-gearbox-can-be-fun-for-anyone