Richard. The ground for the oil light is provided by the bulb socket. The switch I speak of has all 3 of it's terminals electrically insulated from the switch body. The circuit runs power out to the switch and then back through the bulb filament to ground at the bulb socket. The vehicle involved in the question has the bulb sockets grounded as it is a custom wiring installation. Nothing to rethink.
A single terminal oil light switch in fact does provide the necessary ground to the bulb as you state. That's not what we are talking about.
