All that ever should come out of this hose on any engine is vapour. It is part of the polution system. If it is pumping oil you've got a serious pressure issue inside the engine caused by blow by of gases ie: bad rings/engine shot etc.
Was the air filter wet with oil when you checked prior to unhooking this hose?
It seems that filling the engine with oil above the 1/2-way mark of the oil sight glass will sling oil into the airbox, and suprise a bloke when the airbox drain is removed.
Maybe ... BMW made this vent hose too sensitive to collecting oil. Solution => Only fill to the half-way level.