Urgent plumbing issue!

I was taught it was something only done by amateurs. It's doesn't help seal but may lubricate the threads.

Why would putting tape on the olive lubricate the threads?

Why would you want lubricant on the threads? I've never managed to strip them.

Because, by reducing friction at that point you get significantly more axial force, which is what does the sealing, for a given torque. Also, excess torque can disturb the next joint down the line, as discussed in another thread. Same reason you were always taught to lubricate all the studs and nuts when rebuilding a bottom end, so as to get the bearing caps clamped down as well as possible.

Quite, and not sure why you think that. I advocate lubricant on the threads. Perhaps a smear on the olive to allow the nut to turn.

Something I have never achieved. They usually squeak and bind well before then.

I've never had a problem tightening a compression fitting with dry threads. With adequate sized spanners.

As regards an engine, you really need to know if the given torque setting is clean and dry or lubricated. But such fixings are likely far more critical torque wise than a compression fitting.

Yes - tighten until they groan. ;-)

It doesn't - but it might help create a seal if there is a very fine scratch on the outside of the olive that is causing a slow leak.

Mainly when you want to do up a fitting in a confined space and can only get short spanners in there. You can then tighten them better with less friction to fight.

(Jet lube also works - although is a tad more messy!)

Indeed, try tightening the nuts on a 22mm butterfly valve in the gap between the top of a kitchen cabinet and the ceiling, when the only thing you can get in there is a pair of these: