most likely cause of "disappearing oil" is the CCV system. it can magically turn oil into smoke! if its plugged it can turn ALOT of oil into a hydrolocked engine, and usually that renders a rather expensive piece or garage art in the form of a destroyed block (or at minimum a bent connecting rod!)
look for the tell tale "mayo" on the inside of the oil fill cap.

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2003 X5 3.0i (02/2003) sport package, Sapphire black metallic / Black Dakota Sport seats. compass mirror retrofit, trailer hitch retrofit, Prodigy P2 trailer brake controller installed, PDC retrofit, Lumbar support retrofit. heated rear seats retrofitted. Running boards retrofitted and subwoofer install in progress. 4.8is Brakes installed front and back. Bluetooth TCU installed, Rear Fog lights installed. Retrofitted reclining rear seat backs.