Just to give you some insight:
If this is the ZF 6HP Gearbox (I work for ZF by the way), then you can either purchase the oil from TPS/VAG at a premium, or purchase in the aftermarket (same oil) under its original branding on ZF Lifeguard Fluid 6. (ZF Manufacture majority of Audi Automatic Gearboxes) Along with this, you would be best to do a full service with change of filter and seals(can buy a complete kit for typically 150-200 quid depending on spec).
By failing to service any of the ZF 6HP (6 speed) gearboxes, you will be prone to mechanical breakdown and terminal failure of components. ZF officially recommend 50,000-70,000mile service intervals on this gearbox.
However if this is the Audi manufactured box, then id recommend you purely purchase the oil from Audi, since they manufacture the box in this scenario.
If you need any info please just drop me a line, cheers.