Contaminants can enter your gearbox oil as a result of vents or maybe the fill tube through checking/filling. Usually, even so, contaminants are the result of standard wear. The gearbox filter shields against these contaminants which will help make improvements to gearbox lifetime. These usually wear out as time passes. https://filtergearbox69257.activosblog.com/33070459/about-filter-gearbox