Hi mate , changed my gearbox and rear diff oil a few weeks ago , used 75/90 castrol syntrax semi synthetic for the box took 2.6 ltrs
and genuine audi oil for the rear diff 1 ltr . The tool you need is a multi spline drive with a hole in the middle . So you need a special tool ,
got mine from ebay for a tenner ! its an M16 tamperproof spline , try consolidated tools .
Cheers ,
Ant