2002 Chevy Silverado 1500 with 5.3L with low oil pressure

I take it you dropped all the oil when you flushed the pick-up and filled it with good oil. It sounds like you have excessive wear somewhere in your motor. Your fine when the engine oil is cold and thick but once it warms up and thins out you loose to much pressure. It is most likely a worn bearing (s), when you have to much wear more oil escapes through the bearing than designed, when enough wear is present you reach the max flow of the pump (the pump normally by-passes some flow to maintain pressure) and start loosing pressure. Do not run the motor under 10 psi. At this point the motor should be cheap to fix. If you starve a main you risk having to grind the crank, if you starve a rod you risk the same as above but in some cases the rod can seize to the crank and break which will result in block damage. You need a rebuild or a new (used) motor. The 5.3's are very common and fairly cheap if you get a good used runner.

Worth a shot but I think it was in 2005 that GM started putting a screen under the oil pressure sending unit, but I could be mistaken. If there is one under the OPSU it would need to be fished out and cleaned as they get gummed up causing the oil sender unit not to read properly, but like I said than may have started in 2005. Also I have heard on some 5.3L that the oil pumps where not that good, and just needed to be replaced. But I would put a mechanical gauge on and drive it how you notice this problem and verify oil pressure before tearing into an oil pump. And Just make sure that screen is not under the oil pressure sending unit.

You can test/inspect the pump but they don't usually fail like this. When they go it's usually an issue with the relief dumping it back into the pan and you have no oil pressure even with cold/thick oil.

Thanks for your answers. We did drain and replace all the oil when we put in the new oil pan this weekend. It was really dirty and we did notice some metal shavings in there. Didn't know if they were in there from break in or something that was new. We could see up in to the cylinder walls and they looked good. We had read online that there was a screen under the oil pressure sensor but we took a camera and a hook down there and couldn't pull anything out. I don't know if it is stuck or if there isn't one. You couldn't tell with the little camera that we used. We are going to find some dentist tools or something and try again tomorrow cause it seems like it should be in there. We called the dealer to ask about a replacement filter for under the sensor and they had no idea what we were talking about. The engine does have 305,000 miles on it and you may be right that it just has too much wear on it. It has been a great truck. We were looking at replacing the engine but thought since it started strong and seemed to run strong still.. that it was just gunk build up somewhere.

If you have a initial oil pressure reading, nothing up to a reasonable pressure that would mean your free to the oil sensor, if there was a blockage your pressure could rise but you would starve the downstream component of oil. Most parts will not last long after the oil stops. 305k miles is good for a motor you don't know the history of.

how many miles are on truck?

If the oil pressure sending sensor bad you will have low oil pressure an it doesn't have to be leaking oil