IMO the oil Audi chose for the engine plays a large contributory part to the sludge issues these engines face.
If you run semi-synthetic oil, and get it really hot, it breaks down and turns to crap. If you run it for long periods of time (ie 10k) by the time the change interval comes the oil is knackered and depositing gunk all over the insides of your engine. Synthetic oil handles the high temperatures (such as those found in the core of your turbo) much better, and also typically does a better job of cleaning the engine internals.
Personally, i look at the issues these engines have with their oil, and think why take a chance? As a result, I run 5w40 fully synthetic oil and change it every 5k. You can buy Quantum Platinum from VW for about 20quid a gallon, which is small change compared to grenading the motor.
Stating that Trader XYZ use 10w40 oil isnt really any justification that its decent. Semi-synth oil is cheaper, sometimes as much as half the price of an equivalent quality fully synth oil at trade prices. Traders are in business to make money, and when queried, they'll happily justify using it because it meets the specifications that Audi state the engine requires, the same specifications that have sludged up so many 1.8T's...
A rule of thumb i tend to find works quite well is to change the oil when it stops looking golden brown and starts looking black. On a clean engine running good quality 5w40 synth that seems to happen around 5-6k. On a dirty engine it happens much sooner, because all the crap thats coating the insides of the motor dissolves into the oil.
4.5litres inc filter..
