This FAQ about your transmission is useful reading, it's by a guy named loubapache at BW: http://www.benzworld.org/forums/w210-e-class/1463460-mercedes-benz-722-6-transmission-faq.html#post3647154

Fastlane has the Redline D6 fluid that meets MB specs for about $43 a gallon. You'd need one if you'd just drain it and refill, three if you plan on flushing after refilling. (OEM MB branded fluid is about $15/L at the dealer ... or more.) Shell 134 is about half the cost of Redline, google to find a good price on it (around $5-6/USQ).

Probably a whole lot more info than you were looking for but what the hey.