In article , Jack Stein wrote:
Doug Miller wrote:

The joint won't seal unless it's tight, and the tape or dope reduces
friction,
making it easier to get the joint tight. And it's pretty much mandatory for

anything that might be disassembled later.

I don't agree, brass compression fittings can easily be overtightened,
and dope/tape could facilitate that. Also, tape/dope could contaminate
the compression fitting resulting in leakage. Other than that, using
dope/tape on a brass compression fitting is pretty much meaningless. If
the oval ring in a compression fitting leaks, all the pipe dope or tape
in the world will not help it, and a new ring/fitting is needed.

Where have you seen air lines with compression rings in the fittings?

About 30 feet from where I'm sitting? Are you suggesting compression
fittings are not used in air lines? I'm pretty certain I'm not the only
one to use a compression fitting on an air, or a gas line.

Not suggesting they're not used, just suggesting they're not common.

Anyway, if one is using a compression fitting, pipe dope or tape is not
needed, and may in fact be detrimental to the connection. I have seen
compression fittings where pipe dope was used on the joint by someone
who didn't know any better.

As long as you keep it on the threads, and off the ring, where's the harm?