Controversial & Contradictive
The A3/S3 uses a haldex differential to control 4wd(it's not permanently 4wd, only when needed)and require a fluid change, whereas the A4 is a permanent 4wd drive system with a torsen differential. These torsen differentials are pretty maintenance free, I've never heard of a torsen diff requiring a fluid change.
53 A4 2.5TDi Quattro Sport 180 Tiptronic. Heated Buffalino leather, DVD Sat Nav RNS-E, TV, Bluetooth, 6 cd, BOSE, Colour DIS, Cruise Control, Auto lights and mirrors, Interior light pack, Heated mirrors.