I've changed the gearbox oil in my '95 E300D this weekend, using oil to MB spec 236.10. The old oil (also 236.10) had been in for 40,000 miles, and the shifts are now much smoother.
You'll also need a filter, a sump gasket, and new seal rings for the drain plug, and the torque convertor drain plug.
You'll see from this link;
http://bevo.mercedes-benz.com/d/d/en/Spec_231_1.pdf
that your gearbox can use a wide variety of oils, including, 236.1, 236.6, 236.7, 236.9, 236.10, 236.12, and 236.81.
To find some alternative to buying these oils directly from MB, see;
Mercedes-Benz Specifications for Operating Fluids: Overview Sheets
When you're re-filling, put about 4 litres in before you start the engine, and then complete the fill up. If you put all of the oil in without starting the engine (and thus filling the torque convertor), there isn't enough room in the gearbox, and oil will spill out of the breather.
Check also the vacuum system, as a leak can make the shifts harder than they should be.