Also, in my experience in taking off the differential back cover on just one car, a 1982 300D, there is no gasket seal there. Either it was omitted in a previous maintenance action, or Mercedes doesn't use sealing gaskets for that vintage differential.
Recommended sealing method is with RTV Blue sealant. Don't put it on so thick that it squeezes into the insides of the differential - and let it cure overnight before refilling with oil.
But you won't be doing this just to change oil.