Always use some type of lubricant on any mechanical connection. I have
seen plumbers tighten flare and compression fittings so tight they
split trying to stop a small leak. I prefer pipe dope, but tape will
work as will grease or oil on compression fittings. On refrigeration
flare fittings all manufacturers will tell you to use refrigeration
oil, bit pipe dope will work on plumbing flares as well. On threads
use your choice of dope or tape. On ground joint unions use dope on
the faces. Never assemble a mechanical joint dry, the chance for a
leak is much higher.
If I had a nickel for every plumber who assembled a compression or
flare joint dry in front of me and told me that it was the first leak
he ever had, I would be rich!