I have a Defi oil pressure gauge hooked to a Defi control Unit. The oil pressure warning is set at 20psi. After an extended highway trip the oil pressure drops to 20psi at idle which caused the warning light and buzzer to come on. I followed the Defi instructions to lower the warning to around 18psi. But once the car is shutoff and turned back on the warning goes back to 20psi. I have tried everything to permanently change it. Am I missing something here?