I am working to find out why I don't have power at the oil light base and while I was pulling it out one of the spades broke. Is there a source for these?
Yes, the oil pressure switch is the ground in the circuit.Thanks for your notes @deQuincey and @3moons.
It is actually a wedge style bulb and it appears that the design is to pass power through the bulb to the oil pressure switch so both terminals are needed.