Only way to test oil pressure properly is with a known good mechanical guage. That will tell you what the pump is doing. A bad relief valve, which I believe every engine has of some type, will usually fail with no pressure or high pressure. I believe these engines have the pump driven off the crankshaft at the timing cover. The relief valve could possible be accessed externally, but I cannot remember for certain. Either way, anything short of a miracle of the pump/valve failing (which is not common) then the engine will need an overhaul. The only oiling problem that comes to mind to me for the 3.0L engine was upper end oiling/lifter issues. They are not known for other failures in the oiling system.