H aving read the responses of the members, i hope not to piss off anybody with the following. you guyes seem to be doing a little "overengineering" of the effects of the PCV,
In the simplest terms,, the pistons move the same volume of air both above & below. the parallel arrangement of the engines means that: when the pistons " fall " air displacement ( equal to swept volume of the pistons ) creates pressure on the cases, just like a syringe. this parasitic loss is relieved by creating a "vacume" beneath the piston (s),, sucking the air from beneath them.
if not for the " oil bag ", a wet sump engine with this arrangement would blow oil everywhere!, hence the necessity for a remote oil tank. not to take away from the responses here , slightly less dense air in the cases will let the crank spin more freely, but i don't think those are the primary concerns. aerodynamic drag is the nature of the beast, when it comes to a spinning crank. more important to evacuate the air from beneath the pistons eh? that is how the bhp is increased. or do you think it necessary to externaly polish the crank to reduce hydronamic loss too?