It is a switch.. probably ( NC ) closed to provide a dash lamp when key is on. Use a test lamp.. hook one side to power, put the probe on the sender spade if it is a single spade job. Lamp should be on when tractor is off.. and lamp should go off when you start and oil pressure comes up. If not.. sender is bad or you have no oil pressure.
You can take the wire off and ground it and your dash lamp should light if that circuit is working.
If it is a 2 wire job.. it may function as an inline switch.. don't see too many o fthose.
Soundguy