1995 GMC Sierra z71 oil pressure dropped- need advice on this

I would say your heads have overheated and are likely warped. I would shop around for another engine.

The engine will fit into the chasey but i don't think it will line up with transmission and your going to have electrical issues bacause the years are to far apart OBD 1 SYSTEM to OBD 11 SYSTEM is going to be a problem

This is a conditions usually associated with high milage motors.
The worn engine bearings are the usual cause.
A higher viscosity oil can help some.

You may have a bad oil sending unit this is located under the hood near the oil filter two wires are connected to it. i would change this and see if your problem persists, if so you could have an oil pump you wont hear anything until its too late. 20 psi is all you need to keep it alive. good luck

have it tested using a manual gage if still low then main baring ore most likely bad that would mean a over hall if is really low and bearings bad should be making rattle noise when started up first few seconds

I have a 2002 with 130,000 and it ticks,but goes away after 2 min,if yours continues to tick you may have a rocker arm that is getting lose or a lifter that is not pumping up.

could have a cracked head and blown head gasket blowing water out the exhaust

It is on the top backside of the engine block, behind the intake manifold mounted into the top of the block. You will have to lean up over the engine to see it.

30-40 psi of oil pressure is fine for many vehicles including your GMC. I am assuming that the truck has more than 100,000 miles on it and still has the original engine in it. By rule of thumb you only need 10 psi per 1000 rpms in a typical street driven engine to give adequate oil pressure to keep the bearings from oil starvation. The oil pressure is typically higher when initial start up occurs because it is more viscous due to the temperature of the oil as the engines oil warms up the visconsity thins out and the oil flows more smoothly. The drop in oil pressure is a normal occurance. Above 45 is more than required for a daily driven street engine. I believe that the specs on that engine only list 30- 45 psi.

Most likely it's whatever sensor or sensors are used to determine that oil life calculation.