Since there is no way to filter the air being introduced into the valve cover, dust and moisture can enter the engine and contaminate the oil. When Commiefornia decreed that all cars on the road had to be retro-fitted for PCV valves in the late 1970's, the conversion kits we sold had a fresh air tube that was attached to the "clean" side of the air filter to provide filtered air to the crankcase.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!