I just changed my hydraulic fluid. It does get dark.
Have you changed the filter in the supply tank?
Take off the return line of the tank. Unbolt the metal return line using 2 wrenches then unscrew the plastic top on the tank. Pull the unit out and you will see the filter for the system. Get a new one from Mercedes or fastlane and replace it.
Cosider replacing the filter with a new one and then 1 month later it the hydraulic fluid is that dirty.
If you think it is oil it should show as a film.
Take out a little hydraulic fluid and add a few drops of oil and look at it. Does it look the same in the tank or is the hydraulic fluid in the tank just dark? If just dark then try the fliter.
To take the filter off the top unit. push on the spring holding the filter against the top unit. The spring goes through the filter and hooks on a plastic tab that has a hole it it. The spring end is turned in a U shape so you push on the spring and move the spring sideways to unhook it. Try it a couple of different ways and you'll figure out which way you have to move it to get it to release.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car