I was told the oil is 'sealed for life' too, but no harm in changing it. My 1st to 2nd gear seems better in the cold as a result, and the old oil was pretty thin and black.
I got the genuine stuff, at main dealer parts prices it was about £86 though. Still, it hadn't been changed for 120,000 miles, and probably won't need doing again!
I think it was 2.7 litres for the gearbox, and 1.5 litres for the rear diff, though it was specific and different oil for the gearbox and diff.
AG 051 911 A2 - Gearbox Oil - 3 x 1ltr bottles £14.97 excl VAT each
AG 052 145 A1 - Diff Oil - 3 x 0.5ltr bottles £8.75 excl VAT each
I have the 5 speed box though, so yours might be different, just check, but the parts guy said it was the same for all the quattro's. I had to buy a splined drive socket with the hole in the middle to remove the gearbox drain plug, and used a piece of hosepipe and funnel to refill. Check your fill plugs will come out before you drain too, my rear diff one was a pig to get out!
