You do not need any additives for the Rubi axles. Factory recommends 74w-140.
You can change it yourself, Rubi diffs have drain plugs. Very easy.
The plugs have magnets on them, clean off the metal fuzz.
The 12K interval change is for towing and lots of off roading. Otherwise, you don't have to change it that often.
Also change it often if you submerge your axles in water.