Use Dexron III
Any major brand will do - Texaco, Chevron, etc. Look for Dexron III (sometimes it says 'Dexron III with Mercon') on the label.
I have over 200,000 miles on this stuff in my 300SD (1983) and 560SEL (1990). These transmissions are similar to one another and to yours.
I change it every 30,000 miles, along with the transmission filter - do it every 15,000 miles if you drive a lot in stop-go traffic. Take care to drain the torque converter when changing the fluid - it holds a lot, and make sure to use new transmission pan and torque converter crush washers. Also, change the transmission pan seal while you are at it - you need to remove the pan to get at the filter.
If your fluid is brown, it should be changed now; fresh fluid is pink. You check the level with the engine running, at operating temperature. Overfilling is bad and easy to do.