I asked Audi Customer Service about fixed and variable servicing and was told that fixed servicing is better for cars doing low milage / short journeys as there is more of a chance of oil dilution, hence a more regular change is needed. Variable servicing was best for those doing longer trips when the car is regularly warmed up to operating temperature for a longer period and oil dilution is avoided as the water and volatile compounds from short journeys are boiled off.
They also said that regardless of the type of servicing schedule, the car measures the oil viscosity and if it falls out of tolerance the car will throw up the oil change service light. This could be why your oil service light has come on now as your oil viscosity is out of range.
Hope this helps understand what’s going on.
Oh, and I doubt very much that mastachaz‘s recommendation above is valid. Modern synthetic / long life oil, as recommended by Audi and other car manufacturers, will last much longer than that, unless of course the operating conditions are incredibly extreme, but I’m struggling to imagine what those conditions may be, that justify an oil change so frequently. But if you want to throw money at oil companies / dealers the that of course is up to you.
