Your oil pressure gauge shows no oil pressure, does the gauge move at all when you start the truck?
Does your engine make any abnormal noise?
Have they done a manual oil pressure test with a manual gauge to see if the oil pressure is normal?
If the oil pressure is normal then you may have a bad gauge. There have been allot of issues with the gauges on the GM trucks. Most of the time the gauges will get stuck or stop working.
When the truck stalls, is it while driving at speed of coming to a stop and/or at idle?
You need to ask them if they have checked it with a manual gauge. This will let you know if the engine is loosing pressure or the gauge stops working. It sounds like the engine is not loosing pressure, if it was then it would make allot of ticking and knocking type sounds. The gauge cluster can be rebuilt if the oil pressure is good. Here is a website that can help you.
Fist thing you want to make sure on the stalling issue is that the throttle body is clean. IF its very dirty it can cause a stall when slowing down or at idle in gear. If the throttle body is clean, you need to check to make sure no codes are stored in the engine computer.
There are a few parts in the system. There is the gauge, part of the cluster. There is a pressure sensor and the engine computer. The pressure sensor sends a signal to the engine computer, the engine computer then sends a signal to the cluster. If the gauge and the pressure sensor was replaced, then the engine is loosing pressure or the computer is going bad. They should be able to do a manual pressure test to make sure the engine is not the problem. If that checks out, then they should be able to connect a scan tool to the truck that can read engine data, there should be oil pressure data in the engine computer. They should be able to compare actual oil pressure to what the engine computer sees. This should tell them if the engine computer is getting the correct data. If the engine computer is getting good data but is telling the cluster that there is no oil pressure, then the engine computer is bad.
The way the system works is the oil pressure sensor is a on/off switch. If there is pressure the switch is on, no pressure the switch is off. The oil pressure displayed on the gauge is a program set by the engine computer or calculated, not actual readings. Here is some information from GM on how it works:
Engine Oil Pressure Gauge
The instrument panel cluster (IPC) displays the engine oil pressure as determined by the engine control module (ECM). The IPC receives a serial data message from the ECM indicating the engine oil pressure. The engine oil pressure gauge defaults to 0 psi (0 kPa) or below if:
For the 4.2L LL8, the ECM uses an engine oil switch that is open when the oil pressure is normal and closed when the oil pressure is low. At normal pressure, the engine oil pressure gauge will display 275 kPa (40 psi). At low pressure, the gauge will go to 0 kPa and the low pressure warning will be displayed in the driver information center (DIC). At high pressure, the gauge will display 551 kPa (80 psi). Any values between these three values are calculated values determined by the ECM
Depending on what code is stored, he is most likely talking about the evaporative emissions system. This is the system that controls the gas fumes in the fuel tank. The evap canister next to the fuel tank stores the gas fumes from the fuel tank and burns them when the engine is running, This keeps the raw fuel fumes from entering the air. There is a purge valve at the engine that the computer turns on when the engine is running. This purges the fuel fumes from the canister so they can be burnt off in the engine. There are a few problems that come come up on this system. The engine computer checks for leaks in the system, this includes the fuel tank. It also checks the purge valve and a vent valve that lets in fresh air in to the canister. If the purge valve or vent valve stops working it can set a code.
Do you know what code is set?
This code is for the evap purge valve that the computer opens up while the engine is running. If this valve is stuck open, this code sets. The engine computer looks at the fuel tank pressure sensor which lets the computer know if there is a vacuum on the fuel tank or if there is pressure in the fuel tank. If the purge valve is closed, there should be no vacuum on the fuel tank. If the computer sees a vacuum while the engine is running and the valve is commanded closed, it sets this code. The most common cause is the purge valve is stuck open which causes a vacuum on the fuel tank with the valve commanded closed.
It is possible that the fuel tank pressure sensor is bad, do not see this vary much. There is also a code that sets if the fuel tank pressure sensor is bad. You can check the purge valve by disconnecting the eletrical connection and removing the hose at the valve going to the fuel tank. If there is a vacuum comming out of the valve, then its stuck open and this will cause the code to set.