You need to check the breather hoses laying on the top of the intake at the back of the engine. Have to remove the engine cover to see them. They have a bad tendency to fall apart. Need to replace those for sure if they are looking bad, and you probably need to replace the oil seperator.
Do a search on 'oil seperator', this problem has been discussed a lot.

The heater that was mentioned is a heated pad that the dealer sells - for over $400!!!!. Total rip off. Problem can be solved by replacing the above mentioned parts and insulating the oil lines so they don't come in contact with the intake manifold. The intake get VERY cold when driving in cold temps (see this post, I know first-hand: )

http://www.xoutpost.com/x5-e53-forum/...hlight=extreme

Anyways, check out those hoses, that is most likely culprit.