According to the government compatibility list, the A8 (2004), isn't listed:
https://check-vehicle-compatibility-...ufacturer/Audi
Audi
E10 petrol is cleared for use in all Audi petrol driven vehicles, excluding the following vehicles:
First generation direct injection engines:
Audi A2 1.6 FSI, model years 2003-2005
Audi A3 1.6 FSI, model year 2004
Audi A3 2.0 FSI, model year 2004
Audi A4 2.0 FSI, model years 2003-2004
Audi A4 Saloon petrol engine models with OEM parking heater, of model years 2001-2008.
Audi A4 Avant petrol engine models with OEM parking heater of model years 2002- 2008.
NOTE: If your vehicle is listed above you should continue to use E5 petrol. If you are unsure please contact your local Audi dealer.
Should I be worried?