Sorry for the long message below. I ask a technical question at the end, but need to give the background first.

I have enjoyed my 99 Vert for a couple of years. It runs great. Has given me no problems.

I have it listed for sale on this forum as a Mechanic’s Special for low oil pressure. In the meantime, I am still performing diagnostics, given my time, until it sells. (I do not have time or money to tear the motor out and down.)

When I bought it, the oil pressure gauge was pegged as so many of these things are. I have run it from Louisiana to Georgia and to Texas and back like that with no issues or strange engine sounds.
A few weeks ago, I finally had time to pull the manifold and replace the sensor. The job difficulty is greatly exaggerated. It can be done in a few hours and, no, the coolant does not need to be drained. Don’t tear up your hands, don’t cut a hole in your car. Just pull it.
Anyway, on test, I discovered my oil pressure at the firewall sensor was running 15 or less on cold start then dropped to below 10 as it warmed up. I then got “Low Oil Presure” warnings and shut it down. It remained above the GM spec of 6 at idle.
Fast forward much work (new sump oring and oil pump) and checks to now where I have been comparing readings from the Oil Pump Priming Port located on the Drivers Side at the front bottom of the block directly off the oil galley using a Snap On mechanical oil pressure gauge. I get readings of mid-twenties up to upper-thirties of pressure while only getting 15 or less at the (confirmed accurate) sensor up on the firewall.
I have yet to confirm my sump screen is not plugged, but don’t have time to drop the pan - major pain.
My question: I understand the oiling system is not “closed” like a braking system is. A brake system will keep equal pressure throughout with a non-compressible liquid and no “exit points.” As pressurized system open in the end, with the engine oiling system, Inwould expect some pressure drop from the initial pump to the very end of the system (near the gauge) until the oil finally exits the pressurized portion and falls into the oil pan.
I have tried to research to verify this, but haven’t had any success. With a healthy LS engine, has anyone ever gauged pressure from the pump to the sensor to determine if there is a pressure drop? If so, what were the readings?
The standard Melling M295 pressure relief valve is set to 70psi. We typically get 40ish at the gauge. Given if there is a 20ish psi drop across the system, I may be seeing symptoms of a plugged oil sump screen. If that is the case, it may be worthwhile for me to justify pulling the pan, and checking the sump.
I would like to keep my beauty, if I can do it without dumping a fortune (more) into it. Any insight would be appreciated.

 

Doesn't seem like it could be a clogged screen since both of the ports that you are measuring the oil pressure at are after the screen. The main difference between the two points is that the stock oil sender measures pressure after the oil filter. You might try changing the oil filter.