I think if it was electrical you would peg one way or the other. This leeds me to thermostat(s). If Deutz the oil thermostat is in base between oil filter head and block. If cummins in base that goes to top of radiator. Usually thermostat just won't let the oil or coolant warm up much without putting load to the engine but they can become sticky also. Thermostat is the brains at keep engine at proper temp.