I have an 05 suburban 230k miles, 160k are mine.
When I start the engine the oil pressure is okay, around 30 psi. As the engine heats up, the oil drops slowly unit it reaches about 20 psi while driving and 5 psi when I come to a complete stop. Turning on the beeping warning but no check oil light. The oil level is right where it should be.
I replaced the oil, oil filter, oil sending unit sensor, oil pump and dropped and cleaned the pan (not much sludge, looked good for the miles)
After replacing the oil pump the pressure stayed at 30 psi for about 500 miles. Then it started the dropping sequence again.
I used Mobile1 full synthetic 5-30 weight for last 70k miles.
The engine sounds fine, no ticking or bad sounds even at 5 psi.
I have not yet hooked up a mechanical gauge, but I will today.
Suggestions on what it could be? I thought about an upper blockage but the motor looks clean, some varnishing but not major. I could poor some seafoam in the sensor hole.
The 05 does not have a screen below the sensor, I checked.

 

any help?

 

Probably normal wear and tear. Try a higher weight oil. My 93 has 340k on it and I currently run 10-40 in winter, 20-50 in summer to keep the light off.

 

The 20/50 that's been recommended is the easiest way, but, the next time you drop the pan, you might try slipping a new set of Rod and Main Bearings in it. Then the only other probable cause is Cam Bearings.

 

I tried higher weight oil and the pressure dropped even lower, driving pressure at 5lb. I then switched it to 5-20 and it's back to 15 lb while driving.
I made a manual pressure adapter using an old oil pressure sensor by putting a tap through it and screwing in a standard M8-1.0 connector so I could attach my gauge. I will test it tomorrow.

 

I checked the pressure with a manual gauge and .... It is low. Sending unit is correct. So I have a new oil pump as I said above. Next I am thinking there is a problem with the oil pressure relief valve. Does anyone know where it is at? I know there is the one in the pump (bypass) but that should be okay. It looks like there may be another one near the oil filter?