Kev, last year I made the mistake of just adding some gear oil to what was already in the diff. I then drove the car about 1400 miles with that new oil coming out of every conceivable place on the rear end. Guess they didn't mix well. Anyhow, ended up pulling the diff and cleaning the old gunk out of it and replacing it with new 85w-140 as I live in one of the hottest places in this country. With a new gasket, it hasn't leaked a drop.
BTW, that center section is very heavy; be careful.
Steve