All modern cars will require an oil breather system of some sort. In a petrol engine it is inevitable that some gases will ‘leak’ past the piston rings and into the crankcase - turbocharged engines suffer even worse, as the gases in the cylinders are at even higher pressures.
This action of gases entering the crankcase creates crankcase pressure, which needs to be vented – and that’s where an oil breather system comes in.
An oil breather system allows this build up of pressure to escape. If this pressure is allowed to build up and is not vented efficiently it can result in oil leaks from around the sump gasket, crank seals, dipstick tube, and any other engine oil seal as the oil is effectively blown out by the increased crankcase pressure.
In extreme cases some of the old style push-on oil filler caps can actually be blown off!
Oil breather systems allow this pressure build up to be vented. Typically they will included a separator box which will catch as much of the oil suspended in the gases as possible, and then allow this to drain back into the sump.
The gases are then vented off to reduce the pressure. On older cars this simply meant having a breather pipe venting to atmosphere, but on all modern cars the breather pipe actually vents back to the engine’s inlet.
This helps reduce emissions and prevents the car from dropping an oily residue directly onto the road.
But it does mean that instead of consuming fresh clean air, the engine is actually being forced to consume an oily mixture from the crankcase – this can cause issues with inlet pipes, throttle bodies, and other parts of the inlet tract becoming gunked up with an oily residue.