Check out:
[https://www.ralstoninst.com/npt-female-quick-test-adapters]
“They seal due to the "out of roundness principle" which means that the male stretches the female fitting until there is so much force that the connection can hold pressure. One of the challenges with this design is that if you connect stainless steel to stainless steel then over-tightening or poor lubrication can cause gauling and damage to the threads. Thread sealant is needed to seal but only 2 turns of thread sealant is required. ”
And [https://brennaninc.com/brennan-university-old/fittings-101-n...]
“ NPT connections rely on thread deformation- a metal to metal sealing design where the threads of the connectors themselves form together. This design is ideal for single assembly applications and not recommended where connections will be assembled and disassembled frequently due to wear on the threads from deformation.”
I think you’re running across SEO spam. The bane of the internet. However, it is really commonly misunderstood, and I’ve heard all sorts of folks repeat it.
The thread sealant is to lubricate, and can help slightly in low pressure scenarios (like typical gas which is about 1/2PSI or 10-12 WC inches), but it’s to allow the fitting to work, not the primary sealing mechanism.
It’s not hard to do some back of the envelope calculations either and see that has to be true in many situations because the yield strength of PTFE tape or thread sealant (even when cured) is so low. It can’t hold on 100 PSI if it was what was doing the sealing.
Brass, Iron, Stainless? Piece of cake.
That said, there are thread sealants that do indeed provide high strength gas tight seals, loctite makes one for sure.
But it shouldn’t be necessary and isn’t generally in the plumbing aisle.
You can test out the lubrication effect yourself. It’s really obvious in brass, black iron, galvanized, and stainless fittings.
The standard ‘torque guidance’ for NPT fittings, is tighten to hand-tight, then do 2-3 turns.
Try it first without anything (with fittings you won’t mind losing), and the galling and friction is terrible. Often it’s impossible to do 2-3 turns, and hence it will leak. All the force is taken up with the friction on the threads. It may be impossible to undo due to the galling/cold welding.
Lubricating Oil on the threads? No issues, and you can tighten with no leaks unless the fitting is damaged or messed up.
Use tape or pipe dope? Usually even better than the lubricating oil, and minor damage won’t cause nuisance leaks which may require too much force otherwise.
Thin brass fittings and stainless steel are especially bad without some kind of lubricant. The brass even makes a shrieking sound, and is susceptible to cracking.