Loctite 567

I used to be a machine tool pipe fitter and built a lot of high pressure hydraulic systems (3500psi) and we used loctite 567 teflon paste in a tube. Been using the same for 40+ years no leaks yet.

I'll second the loctite 567

Only time 567 has failed me is when using it in freezing services. Otherwise it's used plant wide in our refinery.

Refrigeriant pumps, super light end products that freeze pipes the 567 freezes and eventually leaks. Pretty much the only time we use tape.

No tape for fuel, oil or hydraulic lines. Coolant line fittings are OK.

Why?

Tape interferes with any sort of compression or flare fitting by binding the treads before the mating surfaces have proper contact. Also most tape isn’t developed for the chemicals in most fluids.

Almost any auto parts store has the permatex aviation thread sealer.

Also, small shreads of teflon tape can enter system and cause problems with close tolerance devices. Good example was when I was troubleshooting a low fuel pressure issue on my current boat. Found tiny shreads of tape in the fuel pump check-valves preventing them from sealing. The tape was installed on pipe threads. Both engs had same issue and one eng had damaged piston due to leaning-out @ high power setting. The other eng luckily just shut down.

Agreed, a fitting made to be used with no paste/tape may get leaky from paste/tape

Right, any compression or flare it`s not needed. Don`t have any of those on any fuel, oil or water line on my boat so that`s not an issue.
Also I don`t cover the first couple threads so that the tape doesn`t come off as described.
When used properly tape is the only 100% way for leak free NPT fittings. ive been down this road way too many time using the above pastes. Always something leaking .. use tape, leak solved everytime.