Crawl underneath and on the transmission pan there is a sticker and it will give you a bmw part number, it will probably lead to dexron 6. It may also say it is a lifetime fluid which some people have figured out to be when ever the life of the trans is over after running the fluid way past its life.

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2004 X5 3.0 Auto 140k
2005 325xi Auto 108k
1988 325ix 5spd 279k