So I go to get an oil pressure switch today. We won't mention why I needed one.

Advance only lists the one for the light and Napa is out of both. Go back to Advance and look up a 91 3.1. It lists both styles. Had to drive to the next town to get one so I didn't have to wait for one to come in. Didn't look at it very close until I got home. STUPID! Well it seems some engineer at GM decided to change the thread size. WTF?!? Start doing some research online and find that 88-90 have 1/8"-27 threads. 91+ has 1/4"-18. Nice. It's now getting dark. Sonuvabitch. Go back to Advance and look up 90. There it is. I have no friggin clue why their computers don't show it for 88 and 89 though. But anyway, hope this saves someone some trouble if they ever need to replace their switch.