Why is there gas in oil building up in my airbox tube

The engine is producing more blow-by gasses than the PCV system can handle. 3 solutions have been used to remedy: 1. Increase the flow rate of the PCV valve by owner modification. 2. A separator placed in the hose to the airbox ( a can or jar with In and Out hose fittings) 3. The air flowing thru the crankcase needn't be filtered so route the hose down towards the ground as the air source of the PCV system.

You should put Car's Year, Model, engine type first in order to respond to your question more accurately as possible.

I have a 2011 BMW X6 Direct Inject engine and it builds up some goo(oil+water+dust)inside of intake pipe. I usually clean up once a year but many people recommend to add OIL CATCH CAN to reduce the oil gunk that flows into the intake to burn.