Damn Oil!!

[Monstertrucker]

Positive Crankcase Ventilation is exactly that, it allows for the ever-changing volume of air in the crankcase to be expelled and subequently replaced as the volume changes due to the reciprocating pistons in their bores.

Not quite; it's thermal and gaseous expansion that we're concerned with since the crankcase pressure doesn't change due to mechanical movement; piston ring and stem seal blowby notwithstanding, of course.

As a pair of pistons go up, a pair go down so it's a static volume in that regard.