I had a similar problem myself with my old 300E. I had no oil leak though, so I'm thinking I may have had transmission fluid and coolant mixing.

Anyway, you can flush the system yourself easily enough by disconnecting a couple of hoses and running 'tide' or whatever through there and clean water with a garden hose, take as many hoses off as you can too and either replace, or throroughly clean them.
Then refill with MB or Zerex G05 coolant + water.

I did this and hey presto- no more brown goo!

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys