From your description, it seems the oil is getting up into the heads (where the valve train is), but isn't draining fast enough to resupply the oil pump. The only thing which could do this to an LSx style engine is possibly poor oil changes leaving gunk in the engine. You could pull a valve cover (fairly easy to do) and see the condition of the top end. Not knowing your experience level with vehicles, I'll give simple directions, but you should be able to follow regardless. You can do this from either side of the engine, but the right side of the engine (viewed from the driver's seat) is probably easier to work on then the left side.
- Disconnect spark plug wires
- Disconnect coil pack wiring harness - this is a single connector which will allow the coils to remain plugged in while allowing you to remove the coil assembly
- Remove bolts connecting coil assembly to the valve covers ... you're removing the bolts from the coil mount, not disconnecting the coils from the mount. When you're done, you'll have all four coils still attached to their mount as a single unit.
- Remove four bolts holding the valve cover to the head. These bolts are captured so won't pull out of the valve cover.
- Carefully remove the valve cover looking out for the valve cover gasket
When inspecting, check to see how clean the rocker arms and area around valve springs as well as the top of the head. If these area's are gunky, you'll want to use clean rags and clean up whatever you can. You'll find the oil return passages at the front and back of the head. Some amount of varnish buildup is fine and normal depending on the amount of miles on the pickup, but at 150k I don't think it should be too bad. It's the gunk, like goopy poo inside your engine you don't want to see (like some kind of dark ugly pudding).
To put it back together, reverse my instructions, paying particular attention to the valve cover gasket that it stays in place during assembly.
If the top of the head is clean and looks to be well taken care of, the only other area which may be at issue is the oil pump itself. I'm not sure how it could be at issue, but it is the only other place which could allow for low oil pressure. Usually it would show the issue at startup.
One other thing you might try is to get an oil change, ensuring the proper oil weight is put into the engine. Your engine should take 5W-30. I personally recommend using a full synthetic, and that's regardless of what was used previously. Internet wisdom may tell you otherwise, but most of that is wives' tales. If someone was running heavier oil in it to cover up some noise, there might be some drain back issues, but realistically, I'd highly doubt it.
