I changed the diff oil last weekend. The underside had a good film of oil, and when I drained there was only about 400cc of oil left.

I have previously used PTFE tape on the plug, but that hadn't done the trick. Cleaned up all the threads on both the plug and hole, and refitted with some Loctite Thread Sealer, and then topped up. Haven't used her since, so can't comment if it's been successful1

1972 4/4 4 seater, 1981 MGB GT
1984 Harley Davidson Electra Glide, 1990 Kawasaki ZX10