1997 Chevy Cavalier Low Oil Pressure

Hi. My oil pressure light flickers at warm idle. Like at stop lights but it only flickers and only when I come to a stop after the engine is warm. I have changed the oil numerous times. Upped the numbers from 5w 30 to 10w 30 which didn't help. Upgraded to pennzoil premium oil and it didn't help. The dipstick reads full. Whenever the oil pressure light comes on - the motor doesn't sound any different. No noises, knocking, ticking, etc.

I noticed that the pan behind my oil pan is absolutely covered in black sludgy looking oil. What is the pan behind the oil pan anyways?

Also, Today I was driving and my check engine light came on. Autozone said it was due to a misfire on cylinder #4.

Any suggestions or information would be helpful. Should I change the sending unit. Oil gasket? My brother suggested I put the car out of its misery but it runs so smooth for being an old car. My dad drove it and he said the car rode too nice to be having this kind of problem.