A more involved service, or Audi tune up, is highly recommended at 100k miles. Unless these items have been replaced recently, spark plugs, spark plug wires, oxygen sensors, the fuel filter and possibly ignition coils should be replaced. By this time, they are past their normal life expectancy and are not operating at peak efficiency. Gas mileage, performance, emissions, reliability, and driving enjoyment are all suffering. By replacing these items you are taking major step in getting the car back up to par and the car will thank you. One area of great importance is the timing belt. A simple search on the internet will provide many examples of why the timing belt needs to be replaced, and what can happen if it is not replaced and fails. The timing belt is extremely important to your engine as it helps keeps everything working in harmony. There are hundreds of moving parts inside your engine and these must all work together to allow the engine to work correctly. Its job is to coordinate the mechanical ballet that is the valve train system in the engine. The intake and exhaust valves must work in perfect timing with the movement of the piston and if the timing belt fails it can cause the piston to come into contact with the valves. The result is a very expensive repair. This is normally recommended between 60k and 80k miles.