I have an 89 Chevy C1500 with about 260K miles. A secondary oil gage was installed some years ago in place of dash gage due to the dash gage not working. About a week ago, the dash gage started jumping around rapidly with a "CHECK GAUGES" red light on dash (video is when it first started). At first it was hardly at all and know it is constant and goes from 0 to max. When coasting or sitting at a light, the gage goes to 0 now. So of course, it has me watching my secondary oil gauge super close. Have noticed I am at 40psi when i first start up. Once warm and running at 60mph+ it sits at around 25-30ish. When I idle it drops to 10psi (picture). Oil change about 900miles ago. oil level good. Thoughts?


Pressure at idle

 

Welcome from Florida. I would replace the original oil pressure sending unit and make sure to clean the screen below the sender. I would also go to a heavier weight oil.

 

Excellent. Will try it.

 

Im not sure I follow everything you are describing. But my 88/5.7, since ive owned it shows low oil pressure when it heats up. Often depending on driving conditions, the Check Gauges light turns on, and sometimes it flickers depending on how fast im driving. Typically at high speed or normal operation at any speed while the truck is moving , pressure is high. Check Gauges light is off. When its heated up to operating temp, if I come to a light or stopped traffic, the oil pressure goes down and the check gauges light can come on. Its been this way for the 16 years ive owned it. Never gave it much thought.
Going back to the other Chevys ive owned, they all done it. In my 52 and 54 3100, there was no check gauges light but the pressure gauge would go down to zero after it was heated up and I stopped to idle.
My 56 didnt have a gauge. Just a light. Same thing. Heated engine, the little idiot light would flicker at idle.I hope this helps.