Checked the oil on Mrs' A4 at the filling station today and it was low. Real low. So I bought a small bottle of cheap oil ('twas all they had - CarPlan 10w-40 Mineral) and filled it to the required level.

Drove about 10-15 miles and the oil pressure light came on.
Stopped the car, and it had gone off so drove it home (2 miles). The light came on again. Once again I turned the engine off and on again, no light!
However, if I take it over 2000 rpm it will light up.

Any ideas what is causing it? It is running ok, no unusual sounds or anything.
Could it just be a sensor? Where is the oil pressure sensor?