It should be stamped on the reservoir cap, in the green band. Maybe yours is covered in dirt/oil. They tend to seep around the cap. It is Pentosin CHF-11S. The fluid is green in color.
Sorry, your cap does not match mine! Hopefully, someone else can help with the grade of ATF oil. Here is a link to the Pentosin at O'Reilly Auto Parts, Pentosin CHF11S - Hydraulic System Fluid | O'Reilly Auto PartsI went back to see how I could have possibly missed that. There is nothing green, fluid or otherwise, and nothing printed other than "ATF Oil Only". It is brown, no doubt about that. It must be ATF oil but what grade?
BMW only uses one cap (which has ATF molded into the top) and adds a green label to indicate CHF 11S, the outline of the missing label seems to be visible in your photo :eek7:I went to my local BMW dealership and was told that the cap will tell you what fluid is needed: mine stated ATF and that is what I got.