A post below reminded me to post this picture. This differential has 115,000 miles and most likely, so goes the differential fluid. The bottom cover plate screw is the drain hole. The screw even has a sealing material on the thread, a soft plastic, similar to RTV silicone. I wanted to see how dirty the fluid was so I used a paper towel to sop some of the fluid as it drained.

Refill is easy using a plastic squeeze mustard container. Remove the muster jar spout and cut out the plug first to allow easy flow of the heavy differential oil. Squeeze the container to force the oil into the fill hole until fluid at the top of the fill hole starts to run out. You will have to fill the mustard container about 5-6 times.
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Tom F. Three 940s. '93 T-Rex now running. Mods, RainX. Lien holder on two more (the kid's). Rust In Pieces, '78 245