shifting to Hyperspace... .
I had exactly the same problem on my 2001 Audi A4 3.0 Quattro Sport that I bought in 2003.
After driving around 50-60 miles as a continuous trip the orange “Oil Level Low” text warning came on the dash board. This is not to be confused with a red oil pressure warning which did not light. Oil level looked fine from the dipstick, but did top up a little to be on the cautious side. I was also aware that too much oil can also damage your engine! The warning went away after I got back in the car and started the engine.
Fortunately the car still had ~12 months warranty, so I took it down to my local Audi dealer and explained the problem. In their infinite wisdom, they advised that the oil level was Ok?!
50 miles further the next day – no surprise “Oil Level Low” and the oil level according to the dipstick was OK. Started the engine again warning gone. Perhaps a duff sender unit I thought? Anyway back to Audi for the day. They then advised that this was a known fault on A4 3.0 V6 engines – problem with software in the ECU.
Couple days later they changed the ECU under warranty and the problem was solved.
Also what I was told by the Audi technician, is that opening the bonnet resets any related errors and they are re-checked a further 50 miles down the road. If you stop/start the engine this counter is also reset. This is why errors flagged up got reset and you may think you have solved the problem. This type of error may not show up on short trips either since you need to do 50+ miles.
(I recently had a look into my ECU area housing, the Motronic box says Year of Manf. 2001, and the ECU YOM=2003.)
So it sounds like you may have this “old” ECU Software problem??