The reservoir is not visible to us - is it saying empty on the computer? Perhaps a faulty sensor?

Also, how do you tell it is full... the last time I filled mine (since idrive was barking), it was just a tube that led somewhere hidden and I didn't know how much to add.

I just checked the manual and all it really mentions is that the capacity is 6.3 quarts.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr