Also part # 51 75 7 188 839 (for L side) in the BMW US Parts software at your local BMW dealer. It is officially called a Pressure Lip Seal. This is for a 2011 X5 5.0i, I would imagine it would be the same part # for other models of that year.

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2011 X5 5.0i, Sparkling Bronze, Premium, Sport, Technology, Winter, Premium Sound, Comfort Access,Heated Rear Seats, Rear Climate, Running Boards, Smartphone/Bluetooth package, 20" wheels