If you live in a cold climate, the condensate that accumulates inside the seperator can freeze the valve shut or open causing the oil to: A. Either be sucked up into the valves, pumping it out through the exhaust in massive quanities, or B. Overpressurize the crankcase causing the valve cover to blow the gasket and pour oil all over the exhaust manifold. Either way, it's a mess, a huge mess. One of the remedies is a new designed insulated valve and dipstick return drain that is large enough to drain the milky substance back into the crankcase so it can have a chance to burn off. Short trips during the winter months only aggravate the situation. I'm almost a believer that the valve should be changed as a preventive maintenance issue because of the potential damage it can do. I've experienced both types of failures on an E39 and an E60. The cold weather kit is reasonable pricewise, until you add the dipstick, for some strange reason they want over $250 just for the modified dipstick tube. One BMW tech told me that using a wiper blade metal insert can be used to clear the muck from the drain section on the dipstick tube. It's about a 5 hour job, replacing the valve. Then there's clean up and plug changes. If the car suffered the latter type failure, then be prepared to take it out on a desolate road for about 30 minutes of hard driving to clear the oil from the exhaust. It will look like an extreme mosquito fogger. One thing is for sure, you won't have any tailgaters....Keep an eye on the oil fill cap, excessive condensation or a heavy coating of something that resembles a chocolate milkshake is a sign the valve is failing.
Hope this helps,
Jr