Contaminants can enter your gearbox oil via vents or even the fill tube during checking/filling. Most of the time, however, contaminants are the results of typical wear. The gearbox filter shields in opposition to these contaminants which allows increase gearbox lifestyle. That is why brands arrived up with interior filters. https://manuelhkihg.weblogco.com/34216469/the-best-side-of-filter-gearbox