This is a very common problem with Chevy vehicles. If you want to the oil pressure back up then you need a new oil pump.

Did you install a stock oil pump or an aftermarket high pressure pump? Install a high pressure pump and when changing oil use 4 qts of oil and 1 qt of lucas oil stabilizer.

Micro electronics. Try hooking your old gauge directly from the sensor on the motor. When a unit measures micro-ohms a little pinch in the wire could vary the reading. some mechanics have clip on gauges they could check it out. It probably isnt your oil at all but a small short somewhere ,you are talking very small volts and resistance. A bulb burning too bright could be enough to give a low reading.

I'm going to disagree a little with some of this. First, you don't say what the oil pressure actually is at idle, but if you have 5 to 10 lbs, you're fine. Chevys tend to be a little low at idle. The pressure should pick up to AT LEAST 30 lbs as soon as you come off of idle, and I'd expect to see at least 45 lbs on actual acceleration. (More is fine.) If you have these readings with a stock pump, everything's ok. Second, electric gauges vary a lot. As Paratrooper points out, something as small as a kink in a wire can throw everything off. The only way to be sure of the actual pressure is to hook up a fully mechanical gauge that can't be influenced by the electrical system. Third, your engine isn't knocking, which is a really good indication that nothing's actually wrong. You don't need an aftermarket hi-pressure pump, and you don't need to spend money on anyone's oil stabilizer, and if your truck runs smoothly, you don't need a compression check. Bad rings or valves (which low compression would indicate) don't have any direct effect on oil pressure. Last, about oil, since oil stabilizers were brought up - All you need when changing oil is a good quality oil that meets today's specs... but remember something! The most heavily advertised oils are NOT what you want. No, I'm not going to name names, but I've lab-tested almost everything out there including a lot of oils you've never heard of, and generally speaking, the more money they spend on advertising, the less they spend on their product. I run semi-synthetics myself, for a lot of reasons. Guys, I'm not trying to be a smart-a$$ or put anyone down here, but I build custom engines for a living. Trust me on this.