Fluctuating oil pressure

It's hard to say. Was it maxed out when the engine was cold? I have a true mechanical Autometer oil pressure gauge and at a cold start it's near 70 lbs, then when warmed up it's at 55-60 lbs cruising and close to 40 at idle.

It is probably the gauge cluster, or possibly a bad ground or voltage fluctuation caused by a bad battery or alternator. I have had the speedometer get stuck or be WAY off before. One time I was driving down the highway with traffic, but when I looked at the speedometer it said that I was doing 120

The stepper motor that drives the speedo is known to have issues, especially in cold weather.

I have also heard of sticking or way off guage readings when the battery or alternator is going bad. Usually it affects the fuel guage, but I'm sure that it could affect the others as well. Typically an excessivly high oil pressure reading would be a false reading, especially if it seems to stick, and change when you turn off/restart the car.