Many, if not all, modern BMW engines equipped with the plastic oil vapor separator will experience a major failure resulting in non-start or at best, oil blowing all over the engine bay.
Moisture always mixes with oil, possibly forming a thick whitish mayonaise when condensing (especially when changing oil on long intervals). In cold climates, that white gunk will freeze in the oil separator (PCV valve), blocking the free air passage between the block of the engine and the head.
If you run an engine dating from the early 1990s (excluding the M20, S14, S38 and M42) it will happen to you. BMW knows about the problem for 15 years. Some BMW technical advisors will even blame the driver for not driving their car long enough and that they are responsible for the condensation...
Part #1 on the diagram (M52 shown)
Have your local dealer print out a copy of the SI B11 08 03.
When your oil separator freezes open - it can cause engine oil hydro-lock and serious motor damage ~ and if frozzen closed - cause a broken valve cover and the leaks you also describe - the above BMW service information bulletin SIB covers E39, E46, E53 X5, E60 5, E85 Z4 w/M54 motor