I usually use standard 80-w-90 gear lube.
The D30 should have a plug in the diff cover. Pull the plug out and fill it until it flows over the bottom of the treads, and put the plug back in. The amc 20 should have a plug in the cast housing. It's been a while since I sold the cj, so I don't quite remember which side it's on. :? But the filling procedure is the same.
