I agree with Bulk...make sure you know that it is coming from the pump. In my case I cleaned the engine really well in that area and then kept an eye on it over the next couple of days with an inspection mirror. I saw a drip coming right off the vacuum pump. As far as changing the O-ring or the pump...the O-ring will run you about $8 and the pump will run about $375. I had 2 X5s at the time...a 4.8is and 4.4i and I had to change the pump on the 4.8is and only had to change the O-ring on the 4.4i.

On the 4.8is what it ended up being was that the pump was worn and was leaking oil into the "dry" side and the leak was coming from the dryside. You will see that when you take the pump out, you can take it apart with 4 torx screws...the only seal between the dry and the wet side is the mechanical clearance, which I don't think you can do anything about. You can see where the casing comes apart in one of the pictures that I posted...right below my thumb. I ended up getting a new pump. What I would recommend is trying to replace the gasket since that is the cheapest thing to do. I would clean it up really well so that you can see where the oil is coming from if it starts up again. I had to use an inspection mirror to figure out that the oil was not coming from the 0-ring between the engine and the pump. There is another 0-ring that you might be able to replace when you take apart the pump...but it appears to be a special shaped o-ring.