Q&A

 Master

You need to remove the oil pressure sending unit and install a direct reading oil pressure tester. Your pressure specs are as follows: well I couldn't find the spec in all the info I have. I would guess no lower than 10PSI at idle with engine warm and at least 40PSI at 3,000 RPM's.
What you read next is some trouble shooting for the gauge in the truck.

Customer Concern: The oil pressure gauge is erratic. With the engine running, with no load applied from acceleration or the transmission in gear, the gauge reads straight up in the middle of the range. When the transmission is put in gear, the reading drops low or will intermittently oscillate between half and low. The sending unit was already replaced.
Tests/Procedures: 1. Pull the cluster forward to back probe the Dark Green/White (DG/WT) wire at terminal 12 of the cluster connector C286 with a jumper.

2. Ground the other end of the jumper wire. This should cause the gauge to read directly in the center of the gauge with the key on or with the engine running.

3. If the gauge still fluctuates when loading the engine, the gauge is causing the fluctuation seen on the cluster.

4. If the gauge constantly reads centered as it should, remove the jumper wire at the cluster and install it on the DG/WT wire at the sending unit connection. Grounding it near the circuit at the sending unit should also cause the gauge to read in the center of the range. This will verify if the circuit is OK, or has an intermittent connection between the sending unit and the cluster.
Tech Tips: The sending unit is an open/closed switch. If is open with less than 6 PSI of oil pressure, causing the gauge to read low. With 6 PSI or more of oil pressure, the switch is closed grounding the DG/WT wire. The gauge reads in the center of its range because there is a 20-ohm resistor in series between the cluster connection and the gauge.