I've just put a pre-built diff in mine. It did comes with a little oil in it already, but not enough for my liking, so stripped and re-filled mine with 80wt shock oil
Sounds a bit thin I know but its all I had !! and anyway only feels a wee bit thinner than the stock oil, which must be around 1000wt ....
First run tonight on polish and carpet at Caldicot ... can't wait !!