Same kinda thing happened to me. I didn't even realize I was low on oil considering the 15,000 mi intervals. As soon as I clocked off 8k, the oil light came on. I took it to the dealer and it seemed that there was hardly anything left in it. I was actually dissapointed considering my 04 nor my 06 3L X5 was never low on oil.

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04 X3 2.5i (Retired)
04 X5 3.0i (Retired)
06 X5 3.0i (Retired)
07 X5 4.8i (Delivered 6/20/07)