It is available from your friendly, local BMW dealer. But apparently not in any bottle with Zerex or Valvoline on the label.

Here is the full list of MB approved antifreezes:
http://bevo.mercedes-benz.com/bevolisten/325.0_en.html

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).