thats the ASD reservoir, I believe you can fill it with Pentosin CHF11 fluid which you can get at NAPA.

Its a pretty nifty system when working - you should be having a warning triangle on the speedo too. It illuminates when the ASD works. My car does not have it but I have seen it work on an E320.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)