Well, according to Audi UK:
RE: 2004 Audi A8 D3 3.0 6-Spd Multitronic, Metallic Silver
Vin: xxxxxxxxxxxxxxxxxx
Dear Mr Echo,
Thank you for your recent contact with Audi Customer Services regarding the above vehicle.
Having verified your vehicles details on our database I can confirm the above vehicle is compatible to use E10 petrol.
If there is anything else I can assist you with, please feel free to contact me on the details below.
Thank you for contacting Audi UK.
Kind regards
Haroon Akhtar
Customer Relations Advisor
Audi UK
Tel 0800 699 888
Email
customer.services@audi.co.uk
Still not convinced