OK so let's be clear, the differential operates in its own oil, the same as every car on the road. The drive shafts are solid and do not share the same oil. At each end there is a steel cup that encases the c.v. joint into which the oil is held in. Its then sealed off with a conventional rubber boot secured by steel clips to make it oil tight, to fill you stand the assembly on its ends to fill and then seal it off , simples . My issue was what is the grade of oil and how much do you need to put in??? Maybe it is the same oil as in the differential casing