PCV Valves and Oil breathers
The PCV system consists of the breather tube and the PCV valve. The breather tube connects the crankcase to a clean source of fresh air, such as the air cleaner body. Usually, clean air from the air cleaner flows in to this tube and in to the engine after passing through a screen, baffle.
The system relies on the fact that, while the engine is running, the intake manifold's air pressure is always less than crankcase air pressure. The lower pressure of the intake manifold draws air towards it, pulling air from the breather through the crankcase (where it dilutes and mixes with combustion gases), through the PCV valve, and into the intake manifold.
The main problem with this sytstem is oil seepage out of the oil breather, from high pressure. This problem is more prevalent with turbocharged engines but still exists with normal combustion engines. It is necessary to properly install a crankcase breather baffle on the valve cover. This baffle is designed to allow crankcase gasses to escape while keeping the oil inside by halting it with a baffle. If the baffle is weak, the oil bypasses it and exits out the breather vent into the PCV system. The net result is oil in your air box. You can either try to fix the baffle, or use an oil canisters to catch the oil before it gets to the filter. Ex :
