If you have a tap and die set it should come with a thread gauge, use that to measure the pitch of the thread on the oil pressure switch once you have taken it out, then get an adaptor with the same thread. Diameter you can measure with a caliper. Measure the pressure switch carefully to see if it is a parallel thread or not, if it isn't go careful putting the adaptor in, it should have the same profile as the switch. PTFE tape cannot hurt, mind you as long as you keep it away from the very end of the adaptor and wind it on clockwise as you look at the bottom of the adaptor.