The only gauges on the regulated voltage are the fuel and water temp. The fsm has a warning about a failed ground at one of the screws on the back of the gauge pcb. If the ground fails, the gauge needle actuating bimetal strips see full battery voltage instead of the 7v they are supposed to see, and permanently warp, and thus read forever low. Pull the cluster and tighten all the screws in sight.

The oil pressure gauge behavior is puzzling. Cold start oil pressure should be quite high, as the oil is pretty stiff, and thins as it warms up, so the hot oil pressure is lower. I'd replace the sender and clean up/tighten the wiring connections, or even better, put in a mech oil pressure gauge.