By design the vents in the 216's valve cover admit fresh air when the engine is running and this air along w/the byproducts of combustion exit out of the engine via the draft tube. It basically drafts from top to bottom. Later model engines reversed the draft to both enter and exit from the top.

When the PCV system was 1st introduced in the Stovebolts the PCV mounted in a modified draft tube and the fresh air intake routed from a hose vent on the valve cover that was routed to the air cleaner so the air was filtered before it entered the engine.
I've also seen vented oil fill caps be the fresh air intake.
The PCV system I have uses the 3 vent valve cover and vented oil fill cap for fresh air intake.
When I 1st got my 216 running the engine had so much blowby that I was getting smoke out of those valve cover vents at all engine rpm. The rings loosening up and sealed better along w/the PCV system I added has almost completely done away w/the smoke and blowby exiting out of those vents.

ICE designers want the inside of the engine to be a lower pressure than the ambient to minimize internal pressures but when the engines wear the blowby increases and gets somewhat compressed in the crankcase via the downward stroke of the pistons. Normally these pressures aren't enough to blow through seals and gaskets but running oil levels higher than recommended and normal wear and tear will start spitting oil out. A blocked draft tube and or no internal drafting can cause it also.

Anyways pressureized air might have been too strong a term to use for the air/gases that get compressed in the engine block, perhaps "slightly" higher than ambient air pressure would have been better.
Dave

1949 Chevy 1/2-Ton 3100
1949 GMC 1/2-ton 100
In the Stovebolt Gallery
More pix on Photobucket
Video of the 216 running
Video of a Winter Cruise