I believe the oil pressure sender grounds it's self to the block. This wouldn't be the worst until you try to add a gauge into the circuit. If you have a gauge, then it will also read the resistance in the hour meter. I'm thinking that for the few times that it's going to be left on, that it's not worth the trouble to rewire it. Besides, maybe your meter will stick like my old one so that when you sell the boat it has fewer hours on it than when you bought it!