You wouldn't want to glue a sensor in a pipe, as the sensor should be removable in case it needs replacing.

You'd want to be certain that the joints won't leak. I honestly don't know what they use to join pipes together for oil heating systems. For water or air pipe threads, I just use Teflon tape.

One thing I didn't include in my crude drawing is some sort of shutoff valve before the tee; without a shutoff valve, you'd have a heck of an oil spill if you needed to take apart or service the plumbing. An oil spill would be a very un-good thing.

 

Sorry for being unclear. I wouldn't glue the sensor into the pipe, but do seal the wired connections or use waterproof wire nuts.

Pipe dope is typically used for fuel line connections be sure to get dope rated for fuel lines. There is also a tape product like Sgt.Wookie is talking about. You do not want to use the standard white teflon tape for fuel lines. There is a yellow colored, PTFE tape for sealing petroleum carrying lines.