KirkA

Attach a shop vac to the exhaust pipe (duct tape) and put pressure into the pipes. Spray soapy water at every joint. If it bubbles up, have found your leak.

I have a 2007 with 5.3l. Milage is just past 261k. Going to work yesterday chime goes off and display says"Low oil pressure. Stop engine.". I pull over and turn off the engine and check oil level. Its was good. Started it back up and did not hear any problems and pressure was about 30 psi. Went about a mile and the pressure was gradually falling. Eventually got back to zero again. Got it home and replaced the oil pressure sending unit. Issue still persists.. Next pulled out the little filter under the OPSU. I thread a 6mm bolt into it and pulled out the little filter. Came out easily. It was pretty clogged with crud so I cleaned it up with Brake Kleen. Put it all back together it is all good now. Should have cleaned it before replacing the sending unit.... but oh well. Glad to be back on the road.