For the NON-IRS VS Commodores, you need to remove the diff backing plate, but if you have an IRS Commodore, you just need to undo the middle bottom bolt!
13mm and 15mm if my memory serves me correctly (they changed the size half way through the series)
and the filler plug is 1 n 1/8" in size!
good practice to undo this first, before removing the drain plug!
diff oil should always look clean, as its in a sealed enclosure, and theres no substance that can contaminate the oil, such as petrol
i still change my diff oil every 10thou, for the simple fact is that it keeps everything regular, conditions the seals etc and its easy n cheap to do!
ive also got a limo diff, so changing the oil makes more sense too!
ensures i get 2 wheel spins everywhere i go too!