Crankcase pressure

340 mopar and moparteacher have it right, the motor should not be
expelling oil throught the breathers on a tight motor.That would indicate
ring issues or perhaps exhaust valve guides ( does the engine smoke on heavy
accell ? ).

As for the vacuum issue, 340mopar hits it on the head, an engine does not suck air in. It opens a void that is filled by atmospheric pressure. Unless
it is supercharged, turbo, or more than 100% volumetric efficient due to
a proper build and great exhaust scavenging. The same goes for your body,
you dont really inhale ,you open your lungs and they fill . The lower the
elevation the bigger the column of air above you. Thus at high elevation
you get less O2 because your getting less air shoved into your lungs.