I think there is a bit of a misunderstanding here. Not quite sure what an "oil filter lifter valve" is, but it's not accessed via the pressure sensor. If anything, this would be accessed from behind the oil filter housing. This is on the lower half of the engine. The oil Pressure Sensor, as stated above, is top, left, rear of the engine, and the Intake Manifold needs to be replaced to access it. If they were going to go through all that work, and take a manual reading of the pressure, and it proved good, why did they not replace the sensor? Good question! The oil pressure sensors on GM 8-cylinders have always been known to be fairly common fail rate items. Over time, the internal resistance of the sensor tends to fail, and usually when the oil is hot and thinner, it reads the wrong pressure.
Here's a bit of insight as to how the oil lubrication system works (give you an idea of how the system works, and the components within):
Engine lubrication is supplied by a gerotor type oil pump assembly. The pump is mounted on the front of the engine block and driven directly by the crankshaft sprocket. The pump gears rotate and draw oil from the oil pan sump through a pick-up screen and pipe. The oil is pressurized as it passes through the pump and is sent through the engine block lower oil gallery. Contained within the oil pump assembly is a pressure relief valve that maintains oil pressure within a specified range.
Pressurized oil is directed through the engine block lower oil gallery and through the oil filter tube to the full flow oil filter where harmful contaminants are removed. A bypass valve is incorporated into the oil filter, which permits oil flow in the event the filter becomes restricted. A second valve, the active fuel management oil pressure relief valve is incorporated into the oil filter tube. The active fuel management oil pressure relief valve limits oil pressure directed to the upper oil galleries and valve lifter oil manifold assembly to 379-517 kPa (55-75 psi)
maximum. When main oil pressure exceeds 379 kPa (55 psi),
the active fuel management oil pressure relief valve exhausts
excess oil to the sump.
Oil is then directed from the filter to the upper main oil galleries and the valve lifter oil manifold assembly. Oil from the left upper oil gallery is directed to the crankshaft and camshaft bearings. Oil that has entered both the upper main oil galleries also pressurizes the valve lifter assemblies and is then pumped through the pushrods to lubricate the valve rocker arms and valve stems. Oil returning to the pan is directed by the crankshaft oil deflector. The oil pressure sensor is located at the top rear of the engine.