Contaminants can enter your gearbox oil through vents or even the fill tube through checking/filling. Most of the time, however, contaminants are the results of regular wear. The gearbox filter shields from these contaminants which assists make improvements to gearbox daily life. Aged automated transmissions normally have metallic mesh filters. https://filter-gearbox28258.59bloggers.com/34541818/filter-gearbox-options