right first off dont get drawn in to the old fantasy about emulsified oil on the filler cap .......i work for bmw and i can tell you that 90% of the cars i service show this its quite normal especialy if the car only does very short journeys iv even seen it on a brand new pdi so it proves nothing .. the other thing i can tell you is iv been with bmw now for about 6 years and i can count on 1 hand the number of 6 cylinder engines we have had to take the head off because of the gasket being blown it just doesnt happen that often they seem to be ....well kinda bullet proof lol .
The only problem we get from them seems to be external ::: water pumps thermostats .... sensors ......maby a VANOS but even theese seem to be prity robust
The sauce of the smoke you saw is most likely to be water trapped in the exhaust after being stood idle for a time
Let me know if you think of any other problems you may be aware of ok m8
cheers for now