Its not sealed for life,Mercedes made a big mistake,now they advise chage fluid and filter every 80,000 miles.
The shifter has bushings which might need new ones.If its a 722.605 transmission,things like the conductor plate,might need changing.Or the wires go thru a rubber sleeve,that sleeve can go bad causing trans fliid to wick up,into the electronics.If you can't replace these things,a independant garage should be able to do thoose easy jobs.
Check Benzworld forum under w140 cars for more advise.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt