Maintenance on the A4 was covered under the Audi Advantage Program. Intervals were 5k for the initial service, then every 10k (or annually) thereafter. I think the sludge scare was exposed in the early days of this car's life, thus it’s very likely that Castrol 5W40 was used since the first service.
The coilpacks seem to be another weakness in this vehicle. A set of the latest coilpacks run around $100 from ECSTuning. A failed coilpack may render the vehicle inoperable. Some people have made it a habit to keep a spare set of coilpacks in the trunk.
The timing belt tensioner, timing belt, and water pump MUST be replaced every 60k miles if you want this vehicle to last. The timing belt tensioner is infamous in this vehicle for early failure and will destroy the engine since the 1.8T is an interference design. The water pump also has a plastic impeller (may’ve been corrected in 04? ) and is also prone to failure. A timing belt kit from ECSTuning runs about $300 and includes an updated belt tensioner, water pump, rollers, timing belt, drive belts, and thermostat. There are excellent DIY guides on VWVortex that would allow any determined individual to tackle the job in about six hours. Otherwise, be prepared to shell out around $500 in labor plus parts at a shop.
4L of the Esso LT71141 fluid and a filter kit runs about $100 shipped from ECSTuning. Just measure the amount you drain out and replace with the same amount. DIY guides are also available online.
