Any water vapour in the oil is evaporated by the heat of the oil in the tank and engine. But the mist that goes to your chain case is cooled so water condenses out. Cars in the days of carburettors were always clogging up their air filters with white oily rubbish because the engine breathers were fixed to the air filters.

Breathing into the primary chaincase was a mod carried out on at least one pre-war works competition machine with a similar set up to the WM20, so presumably thought to be a good idea. Moisture will inevitably be present as explained by Ian and also because water can get in at the back of the clutch. My remedy is to regularly monitor the state of the chaincase oil and if it becomes contaminated drain, flush and replace. Oil quantity is fairly insignificant and if you really want to economise use engine oil from your last change. Lubrication is only for the primary chain and the cush drive. I've done this for 40+ years with no problems.