Another advantage of having a controlled (somewhat) pressure in the crankcase is improved sealing of some piston rings.
I say some because even though many modern rings are expanded partially against the cyl walls by the pressure differential across the ring, older ring designs depend on the spring tension to expand them.
For gas expanded rings to work best there should not be much positive pressure below the ring and the PCV can evacuate the crankcase of blowby and keep a more uniform pressure
This is a delicate balance of having a system that can move just the proper amount of air to keep things right and has to take into consideration the pumping action of the pistons below the rings.
A 4 cyl with the pistons moving up and down in pairs but at opposite times tries to move a lot of air in and out of the crankcase while a 6 cyl or 3 cyl has less pumping action because of the piston movement being 120 degrees apart.
Most 1 cyl small engines pump so much air below the piston that their crankcase vent system has a check vlve in it that allows air out but none in--they have no fresh air let in.If the crankcase gets unsealed at dipstick or a missing bolt the pumping action will cause oil overheating and increased oil loss at many joints.
So a well designed crankcase vent system can help ring life, oil control, and compression control by letting the rings work more at a optimum.
The horsepower seekers have carried this to a high level with exotic design of gas operated rings and pump evacuated crankcases.
My seat-of-the-pants dyno can't register any difference in power output with or without PCV but the oil leaks sure do pickup when the crankcase is positive pressure.
