Changing the transmission fluid and filter is probably the MOST IMPORTANT service that you can perform on your A8. The transmission has metallic wear components. Once these wear, the filter clogs up and the oil is degraded over time. This starves the hydraulic pump of oil, especially on cold start ups and causes the pump to cavitate. This is the squealing noise that can be heard. This can cause long term damage to the transmission.
Audi states the automatic transmission fluid (ATF) is a lifetime fluid. This is not true. It is recommend to change the fluid and filter every 25,000 to 40,000 miles. This can save over $8,000 for a rebuilt transmission.

Special Tools Required
There are a few special tools required to complete the job. There are two pan torx screw above the right hand catalytic converter that will need to be eased out with a modified torx wrench. You will also need a breaker bar (2′ pipe to fit over the allen wrench) most likely to remove the fill plug, and a pump to get the new fluid into the transmission.

Download Audi A8 Transmission Service