Isn't Dextron mostly for American cars and cars using GM gearboxes? Anyway Audi part number you want is G052162A2 for 1l bottles ( you can have different capacities just change last character to 1 or 6 ect.) You can search for the Audi number and cross-reference.
There are about 7 litres in 4 speed box but you can only drain about 4 from the sump without removing torque converter or flushing it.
This is supposed to be half decent, cheap and compatible with G052162A2 but I have no personal experience so use it at your own risk.
http://stores.ebay.co.uk/oel-guensti...sid=1101316816