Grounding the instrument voltage regulator

"No, none of them work" Here's a thought... The wire terminal at oil senders rarely have a insulator on them. If that bare terminal falls off and shorts to ground, IT WILL FRY the oil gauge, limiter, or both. An oil warning lamp doesn't apply.
Temp sender wires normally have their insulator on the terminal but that wire is sometimes pinched under a valve cover or otherwise shorted,
again frying that gauge, the limiter, or both.