You get air in the system when you over or under fill the tranny. Either one will cause the oil to foam and cause hard shifting. Under filled, the system could suck air on hard turns. You will know it because the car will neutralize for a second or two. Over, the gear sets will stir it up. It works it way out. There is anti-foaming agents in the fluid but they only do so much. The tranny is vented, otherwise it would blow out the seals when it heats up. I've seen folks overfill the tranny and find red puddles under their cars later.
