I have a 2003 Chevy Silverado SS with 6.0 engine. My truck has 40 psi until engine warms up then drops to 20 while moving and 5 in idle. So I had a new oil pump put in. Worked better but same problem. Replaced the sending unit twice in the last two months. Had 4 different garages test everything. No one can tell me why it is still doing this. The engine runs smooth and quiet. HELP

 

you either have a pressure or pressure reading problem. a master oil pressure gauge reading at the sending unit or oil filter housing would confirm a pressure problem.


old tired engine is one possibility. when oil clearance increases due to bearing wear, the pressure drops. the pressures indicated make me believe this is the case.
either run a thicker oil or have the engine rebuilt or replaced.
minimum oil pressure requirement is 10psi per 1000rpm. the oil pressure warning light probably comes on at 4-5 psi.