When the dealer services your car, they are supposed to reset the oil or service interval warning light that comes on in place of your trip odometer when you start your car. It can be temporarily reset, by pushing the odometer reset. But when the ignition is turned off, the service warning will reappear when you restart your car. To reset it permanently, follow these instructions:

1. Insert your key into the ignition.
2. Push in the odometer reset button, hold it in, and turn the key to the first click.
3. The display should read either “oil” or “service” if it does not, repeat steps #1 and #2.
4. To reset, pull out your instrument light dimmer button at the bottom of the tachometer.
5. Turn the ignition to the off position. When the ignition is turned back on, the display should be back to the trip odometer.

Note: This procedure has been confirmed on ‘98 and ‘99 model year A4’s.

http://www.audiworld.com/tech/elec.html

When the dealer services your car, they are supposed to reset the oil or service interval warning light that comes on in place of your trip odometer when you start your car. It can be temporarily reset, by pushing the odometer reset. But when the ignition is turned off, the service warning will reappear when you restart your car. To reset it permanently, follow these instructions:

1. Insert your key into the ignition.
2. Push in the odometer reset button, hold it in, and turn the key to the first click.
3. The display should read either “oil” or “service” if it does not, repeat steps #1 and #2.
4. To reset, push the clock adjustment knob the bottom of the tachometer.
5. Turn the ignition to the off position. When the ignition is turned back on, the display should be back to the trip odometer.

http://www.audiworld.com/tech/elec.html