:stupid: bp405 is correct. The tube that vents into the airbox from the engine does just this!
Edit, You replied while I was typing.... Engines build preasure while running. The engine is like a large vacume pump from the bottom side, that sucks and blows with with every revolution of the piston(s). Now add in the oil pump preasure and now your creating and moving more air volume. This preasure has to go somewhere and is releaved into the airbox. I wouldn't advise removing the dipstick while the engine is running.... as it may blow oil out at you like a whale blow hole! Only check the oil with the engine off.... and allow it to drain back down into the tank/crank case to reflect the propper oil level.
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