Like others, I've used Permatex #2 for many years on many, many threaded connections with no leaks. I have also used Permatex Thread Sealant with PTFE and it works great; for NPT fittings this product makes for a very good seal, yet allows for easier removal if ever needed and it does not harden.
One note about AN tapered fittings - use a couple drops of assembly oil (air tool oil works well too) wiped on the face and threads will allow you to fully assemble by hand while reducing galling potential. And then, once tight by hand, tighten one more 1/4 turn with a wrench and you're done - regardless of the fitting size and no torque wrench required.
- RV10, N1530G (reserved). Empennage in progress.
- RV12, N975G, "The Commuter"