I would imagine either an o-ring or metal gasket/washer.
Thanks for your input.
Yeah... I was thinking a copper ring gasket, but I don't have anything in my kit that will fit. If it's an oil pressure sensor I would think there needs to be a very tight fit.
MY LESSON... is that when I was removing all these sensors and plugs, I should have documented if they had gaskets and what kind, AND photographed or video recorded these parts on the engine!! I screwed up on that!