heater valve thread sealant

Okay, this is based on my personal experience and should be considered an opinion.  When I first started using the 565 I'd tighten fittings as I had with tape.  That was a mistake.  565 is an anaerobic sealer, and cures in absence of air so it would cure in the threads.  I ended up with some fittings I could not disassemble without destroying the fittings.  Once I learned that I wasn't torquing lug nuts, I have had much better success.  As White70JavelinSST pointed out, you have to remember how pipe threads seal, the threads are tapered so that you end up with metal to metal interference which provides a seal.  In a perfect world this works fine.  But if either thread, male or female, is degraded, you will probably have a weeper.  565's strong suit is that it fills in damaged or degraded areas of the threads and gives you a leak proof seal without reefing on the fitting.

If you are uncomfortable using an anaerobic sealer (there are others beside 565), by all means use something else.  Whatever you do, chasing the female threads in a threaded NPT joint is always a good idea.

As to what did the factory do?  I have no idea.  As an example, if I get an OEM threaded part from a dealership for a current automobile like a temperature sensor or a oil pressure sensor, there will be sealant on the threads.  Aftermarket, sometimes you get sealer, sometimes you don't.  Here are two examples.

With sealer.

Without sealer:

I don't know if this helped or just muddied the waters but it's my best shot.