oil warning light

[acp]

Punto 1999. When i start car up oil light goes off as normal,but after about 5 minutes of running (stood still on drive) oil warning light comes on, but goes off if you rev engine up,the longer you leave it running the more revs you have to give for the light to go off.Oil level is ok.

[Toureman]

When was the oil and filter last renewed? is it the correct oil? check this first.