Interesting comment Andy. British bikes for decades had a mesh "strainer" in the sump for the oil scaveng system and a "strainer" in the oil tank for the supply system. These worked perfectly by preventing any metal being scavenged and if that did not work then a second filtering process occurred in the tank.

External filters were fitted to a few bikes in the '70's but not all. The original two "strainer" filtration coped with Monograde oils, Multigrade oils and even racing grade Vegetable (Castor) based oils. Simple to make, simple to install during the build and simple to maintain. Then the Japaneesy bods made it all relatively complex.