I used generic fluid from the auto parts store when I did mine. It has been over a year now and no problems. Flushing is way more beneficial than simply evacuating the reservoir. Not sure if the W210 is more picky about fluid, maybe another owner can chime in. You will also want to change the filter but I remember reading somewhere that the filter is part of the entire reservoir assembly requiring you to replace it entirely.

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1989 300CE