Actually, NPTF (National Pipe Taper Fuel) is suitable for hydraulics. NPT is not.
Of course, discussions like this are common, where a lot of people are talking about NPTF, but saying NPT. Sometimes people are using shorthand, and some of them may not be aware that there is a difference.
NPTF is a tighter-fitting version of NPT. The threads are cut in such a way as to provide more interference and seal more tightly than standard NPT fittings. If you are using high pressure steel fittings, they are NPTF, and not NPT.
NPTF is sometimes called American Dryseal Pipe Threads, and doesn't actually require a sealant when you're assembling two new, unused fittings. Problem is, though, that once assembled, the threads become deformed. If you take that assembly apart and put it back together again, it won't seal as tightly as it did the first time. A lot thread sealants (like PTFE paste) aren't really sealants as much as thread lubricants, to reduce the amount of deformation and damage that occurs when the assembly is tightened.
This is probably why almost everyone I speak to uses some kind of thread sealant with these fittings anyway. It wouldn't be necessary if you were assembling the fittings once and never planned to disconnect them again. But that doesn't happen often, so in preparation for that future re-use, it's a good idea to apply some PTFE paste or other type of sealant.
Because the basic thread dimensions are similar, NPTF fittings can be used anywhere NPT is required. However, if NPTF is required, do not use standard NPT. Always buy hydraulic fittings from a hydraulic supplier and not from Home Depot. Your hardware store may have plenty of NPT fittings, but they are probably not suitable for hydraulic pressures.
The other fittings being discussed, including JIC, ORFS, ORB, and BSPP (parallel) are all straight threads. They all use a different kind of seal, and DO NOT seal on the threads the way they NPT, NPTF and BSPT tapered threads do. That is why you should never use a thread sealant on those other fittings.
