Just changed the differential fluid in my '83 300D (110,xxx miles). Both the drain plug and the fill plug appear never to have been disturbed, and were VERY tight. The fluid that came out was a brownish red and had some bubbles/foam in it. It's possible that it never has been changed. I refilled it with the Mobil 1 synthetic gear oil. While I had the plugs out I soaked them in a bit of brake cleaner and then cleaned the threads with a wire brush. They were a little gummed up with black stuff.

My problem is that the differential drain plug won't go all the way back in. I can screw it in some by hand and then some more with my socket wrench, but once it's probably 2/3 of the way in it just won't turn anymore. I tried removing it completely and trying again, but never could get it to go in all the way. I'm fairly certain that it was screwed all the way in before, but I didn't make a point of noticing and it was so gummed up and dark that I might be wrong about that.

Has anyone else had this problem, and do you think it's anything to worry about? I took the car for a little test drive after I finished the change and then parked it in the driveway about an hour ago, and so far there are no leaks.