The low oil pressure warning is a common occurence. I'm in the middle of trying to diagnose mine right now! There are a lot of posts on the A6, 2.7t, and S4 boards about this, but none really root out the cause. I just changed out my oil pressure sender ($12 part)yesterday and instead of intermittent low oil pressure warning it's now on constantly although my pressure checks out fine. WTF? Next step is to drop the lower pan and check the sump screen.

Changing the oil pressure sender is a bit of a PITA. You have to drop the sway bar, move the intercooler hose out of the way and use some contortionist movements of your hand to get in there. I have an Audi pic of it if you want... Some people have had luck just changing this as it gets loaded with oil.