Oil Light Stays On

[BriceW at webtv.net]

I am working on a 1990 Audi 100 with 220.000 miles. The oil light stays
on. The oil pressure and temp guages are reading fine. I changed the
oil.
I am working on this car for money as I also will learn about my 91
200TQ20V and use the money to buy upgrading  parts for her.
I cant figure this out.
I will disconnect the battery cables for 30 minutes and then try it.
Any suggestions would be most appreciated and has this been a problem on
the 20V?