Why do I get an "Oil Pressure" warning from my '86 Lincoln Town Car?

I have a 4 door 1986 Lincoln Town Car and it eats oil. I'll put 5 qt in on a Monday and by Friday I'm getting an "Oil Pressure" light on the dash board. It usually occurs after driving the car for more then 20 minutes or being at a slow pace (traffic) or red light. The longer I drive the more consistent I get the light and warning beeps. The car also putters as a result. What is wrong with my car so I can figure out the best way to go about repairing it. If you need additional info or are in the ATL area and could look at it email me @sonovas2@yahoo.com