My oil pressure gauge fluctuates from no pressure to high pressure in idle and when driving.

Hello,

It the gauge fluctuates but you do not have any other indications, like rough idle or noise. Then it is likely a bad sensor or a faulty stepper motor on the gauge.
The oil pressure sensor is located on the rear center of the engine near the top (see diagram).

Another common issue with these trucks is having faulty stepper motor for the gauges. In order to access that part, you will have to remove the gauge cluster.

Here is a helpful video:

https://youtu.be/_HEC44xENxw

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.