If it smells different than oil, check your brakes. You might have a seized parking brake. Easy way is to feel the rims. If they are hot, you may have a stuck caliper or rear parking brakes. The smell of cooking brakes isn't all that pleasant either.

But if it's oil, I'd say check your valve cover gasket. If it leaks, it tends to right onto the exhaust manifold. Some cars (not sure if the 6-cyl X5 has or not) have a heat/drip shield over the manifold. You'll be able to see if it is leaking onto the manifold or not though.

__________________
Daily Drivers:
- 2008 535i, 320,000km
- 2004 X5 4.4, 01/2004 production, 420,000km
- 1997 328i, 350,000km

Track:
- 1996 328i, track/race car, ~300,000km

Winter:
- 2013 Ski-Doo MXZ X 800 E-TEC, trail can
- 2007 Ski-Doo MXZ Blizzard 800 HO
- 2001 Ski-Doo MXZ 600 w/800 engine, exhaust
- 1978 Ski-Doo Olympique 340 (vintage race sled)
- 1977 Ski-Doo Olympique 340E