I would suggest every 30,000 miles. For some M-Bs (like my '96 E300D) there doesn't seem to be a factory spec--the first PO took the car to the dealer for everything and changing the PS fluid was never mentioned (W210 has no filter). It looked to me as if the fluid had never been changed in 248,000 miles -- it was black and the pump was performing poorly at low engine speeds. I flushed the system and will repeat it after a few thousand miles just to make sure.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .343,000 miles Wife's car
2005 E320 CDI . . 148,000 miles My car
Santa Rosa population 177,300 (2026 projected)
Total. . . . . . . . . . . . 668,300
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970