How Are You Using to Seal Threads of Water/Oil Gauge Sensors?

Dana67Dart said
Super easy to tell. Get out your decent DVM and check resistance between the body of the sender and the cast iron or aluminum next to it.

There has been lots of back and forth about teflon tape etc causing a open cir.

Agree with all the above. No matter what you use, just do a quick check with an ohmmeter to make sure your ground is good- it only takes a minute.