Teflon tape was originally developed for using on used pipe threads on plumbing pipe. Prior to the introduction of Teflon tape for use on worn old plumbing fittings, plumbers would use pipe dope and cotton sewing thread in the pipe threads to get a good seal. Using Teflon tape on new NPT fittings is worthless, because the sharp edges of the thread will cut the Teflon into ribbons. Pipe dope properly applied fills the gaps in the NPT, as you tighten the fitting. NPT threads are tapered, and the tighter you twist, the more pressure is exerted on the sides of the mating fitting. In theory, you can over tighten NPT fittings, and crack them. This was a common problem with Teflon tape when it first was introduced, a plumber would use the tape on an old pipe that was being threaded into a boiler, and since it reduced the friction, and the plumber lost the feel of how tight it actually was, the boiler would crack at the fitting, rendering it worthless. Today, you can purchase many types of thread sealant, and some actually have Teflon infused in them. Stay away from anything with Teflon when working on hydraulics. I would recommend Rectorseal #5 for hydraulic fitting connections.
