On modern 4wds, primarily turbo-diesels models, small quantities of gas from the combustion process in the engine trickles past the piston rings and into the sump.
Crankcase ventilation stops this pressure from growing by releasing it on older 4wds straight into the atmosphere, whilst on newer 4x4 vehicles it is recirculated into the air intake to be burned in the cylinders.
This air is heavily laden with oil mist, precisely the kind of thing that we don't need hovering in the air we inhale, nor in the air our engines want to breathe.

A catch can's main job is to separate the oil from the air so the oil can be stored and either disposed of periodically or fed back into the sump where it can continue to lubricate your engine.
They are a passive device that won't alter your engine's fuel economy or performance directly and isn't an illegal modification.
Instead, a catch can will reduce maintenance costs and minimise any potential issues.
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