I have little faith in the "full explanation."
Large breathers, with or without one-way valves, are well known to reduce oil leaks.
The desirability of drawing fresh air through the engine sounds like superstition. Why would I want cold Scottish fog to enter my engine?
Mr Bunn appears to be ploughing a lonely furrow if he purports that a partial vacuum in the crankcase reduces power.
Well he and the engineering school at the University of Auckland where the latter work was done and a couple of graduate thesis would all have to be wrong as well.
Please do not tell HD as they modified their engines and AFAIK rex is still getting royalties for that work.
Have a look under the hood of most Mitsubishi's.
At the rear of the engine in the rocker cover is a pipe from 1/2 to 3/4 on latter models which is plumbed into the air cleaner.
This is the fresh filtered air inlet.
Near the distributor is another line usually 1/2 with a one way valve in it which goes to the carburettor or to the engine side of the butterfly on fuel injected engines.
This is the crancase exhaust
Clean air in , dirty air out.
So if Mitsubishi went to this extent on a partially balanced engine pumping system, are they just superstitious ?
They are however not 360 deg engines like a single or Pommie twin so the flow characteristics are totally different and they can get away without a valve on the intake.
When Phil Irvine developed the Repco Brabham "Yella Terra" engine one of the things he did was to add a fresh air intake and a valve regulated exhaust.
GMH fitted this engine ( minus the big cam & domed pistons ) to the 1966 range of cars.
The full Yella Terra kit came with a new PCV valve and rocker air inlet designed to work properly at the higher revs that these engine did comparred to the standard red series engines.
I know this one as I fitted a head without the modified breathers and it was as smokey as hell under full throttle till I got the right breathers on it.
Oh and Phil did this after he had a stint at Chamberlan Tractors where he doubled the Hp of their entire range ( both petrol & deisel ) and these all got fitted with flow through crank case ventilation as part of the upgrade.
Now perhaps this was because we have high energy air down here comparred to your lazy fog up there, but some how I think not.