I disagree-- Teflon tape is OK and it holds up to a relatively high temperature. The key to success is wrapping the tape around the threads in the right direction, putting on just enough to seal, and not allowing the tape edges to get near the end of the fitting where it can get into the lines. Winding the tape on the threads with a little tension lets it seat down into the threads so it doesn't interfere with screwing the fittings together.
Loctite is also good; they also make a "hydraulic sealant" that I use when I have a fitting that I don't envision ever taking loose again.
As they say "You pays yer money and yer takes yer choice".
Regards, Neil Tucson, AZ