1996 Impala SS Low Oil Pressure Light below 800 RPM

Hello,
 
I have a 1996 Impala SS that the Low Oil Pressure light comes on when the RPM is less than 800 RPM.
 
Some Car Info:
Mileage 157,xxx
Oil: 5W30 (Have used Mobil 1 every 3000 miles since I owned the car from 41xxx miles.  About a year ago I started using 5W30 Amsoil. Continue to change Oil every 3000 miles.)
 
No leaks
No strange noises at Idle or at higher RPMs.
 
Per the Helms service manual the light trips below 4 psi.  I have replaced the oil pressure switch and hooked up a meter/guage.  The guage does not have any markings below 10 psi so I am not sure when it cuts out and goes to 0.
 
Per the Helm manual here is what the mininums shoudl be:
 
RPM                   PSI
1000                   6
2000                  18
4000                   24
 
My numbers meet this and are listed below:
 
RPM                   PSI
1000                   7-8
2000                  20
4000                  25
 
 
The light does not come on right away when the engine is cold.  After the engine comes up to operating temp and the idle comes down slightly the light starts to come on when the RPM is below 800. (Idling or stopped at a street light/sign.)
 
Here are some number I come up with:
 
Engine Temp 200 degrees.
 
In Gear:
 
RPM                Low Pressure Light
599                     ON
800 or higher         OFF
 
In Nuetral Idling:
 
RPM                Low Pressure Light
735                      ON
800 or higher         OFF
 
 
 
Questions:
 
1.) What shoudl my next steps be in troubleshooting?
2.) Should I replace the Oil Pump or could this be something else?
3.) Shoud I change the Oil with 10W30 and see if that helps?
 
Any help is greatly appreciated.
 
Thanks,