For you guy's that grease I would recommend, removing the plug. I myself will change mine out at 20-25000 miles just to see what the grease looks like.
Next, I'll try to find out What grease Harley uses..
Hope this Helps....................
Note; in the service manual it state's: (Never add grease to the differential. The grease added at the factory lasts the life of the motorcycle.)
It looks like were on our own on this one, again if you do, remove the PLUG!