If you're talking about the cover over the oil pan drain bolt, I've replaced it twice and it's missing the very next time. Not sure if my experience is unique or not.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim