'97 E 320 rear end question

I have a slow leak in one of my differential seals that's not necessarily worth fixing yet, but I know I'm going to have to top off
the gear oil. Does anyone know if the standard 80w gear oil is ok for this, or does it require some crazy MB fluid that's more expensive than plutonium? Also, I'm guessing that the plug is a giant hex nut like my '78 w123 had, and if so, does anyone know what size?
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