I don't want to sound annoying to folks with newer models, but on the C4 changing the oil is as simple as in many other cars.
However, guys like Kevin who own C5s and C6s can give you better answers.
Although not for an A6, here's a DIY oil change for a 2008 A4 with filter casing
Also, there's another DIY regarding an Allroad, with the filter as a stand alone
feature, similar to a C4.
'95 12V - 167,400 miles - engine rebuild + ps fluid and ATF change; switched to Mobil1Syn 0W-30 at 158,200; Purchased Dec. 2008 @ 152,100 miles - Presently 168,900 miles