Permatex Pipe Sealant

You won't have a leak with this material. Just don't tighten it excessively or you probably won't get it out. Just a comment, the concern about insulating the sensor is misplaced, Pipe threads are tapered so the thread flanks will always have metal/metal contact. Thread sealant does nothing but fill the voids in the threads.

"We live in a time when intelligent people are being silenced so that stupid people won't be offended".