Series Land Rovers have a very effective oil bath air filter system that was been designed to work in jungles and deserts all over the world in all sorts of unpleasant conditions. If you’re off for a jaunt round the Sahara then the original oil bath filter is the one to keep. The oil bath catches larger particles from the inertia of the air changing direction quickly inside the filter housing, also, with the movement of the vehicle the oil splashs the mesh and this catches the finer particles as the air bounces through it. It is important not to overfill the oil past the level marker as you run the risk of sucking oil into the intake and into the engine which at best would result in rough running and a lot of black smoke, at worst with a diesel you could have a runaway engine as the engine starts to run on the oil. Obviously not enough oil will mean the filter is not working as effectively as it should. Changing and checking the oil in the air filter should be included in your regular sevicing of the vehicle as well as cleaning out the mesh with petrol.