1968 Barracuda 383/4spd

The temp gauge and the fuel gauge are both powered up by the same dash-gauge regulator. That regulator is just an on/ off switch with a heater in it. When powered up, all it does is continuously turn on and off, in a duty cycle that accomplishes an average of about 5 volts. The factory gauges are so slow that they rarely catch the fluctuating.
Of course the oil-pressure sender has to vary it's ground resistance with changes in oil pressure, to make the gauge work...