I flushed my M5 steering system and used Mobil 1 Synthetic ATF. I think it's a Dexron III/Mercon/Mercon V fluid. Works great.

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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