It looks like it's not sold separately for E53 but comes with the tank. The cap itself has the same part nr. on E53 and E6x but the rubber seal is different so difficult to say if that E6x part would work on E53.

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E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey