Well worth googling how an oil air cleaner works very clever, air shoots down the middle towards the oil, then does a 180 deg turn back up through the mesh and dirt is dropped into the oil with centrifugal force, I don't think mesh is actually in the oil.
Any old dirt cheap engine will do.
Worth changing the oil once a year and giving it a good scrub out, mine has been on for roughly 4 years and theres not a drop of oil mist inside the intake pipework, I would have to guess that a tdi flows at least twice as much air as the std diesel engine, which shows you just how good it is.