Bought truck in April. about one month later oil pressure would drop to zero and warning message would display Low Oil pressure and warning chimes would go off. Local GM dealer tested with mechanical gauge which fluctuated same as truck gauge, so they changed the oil pump at great expense. Oil pressure still fluctuates erratically but at a higher level and warnings have not come back on. Problem is still there because the pressure is erratic and weird the way it fluctuates. Cold start gauge goes halfway up, once warm drops below 1/4 on the gauge, driving on the highway at 60 mph when you let off the gas and decelerate for turning, gauge goes back up to about 1/2 way, every time you let off the gas oil pressure increases until the new rpm is reached and held, then it falls off again. 2 shops have tested the sender and gauge and say they are fine, it is an internal engine problem. Don't understand why pressure goes up when decelerating but only momentarily. Anybody had this happen?
See this post, they cover several possibilities including the pickup tube, a damaged pickup tube seal, a defective pressure bypass valvue, or possibly a clogged filter screen located before the oil pressure sensor.