Here is the name of the stuff I was mumbling about on the weekend:

Loctite 567 PST pipe sealant with Teflon.

(high temp, for stainless steel and other metal fittings).

I have found it to be great stuff for pipe thread connections in stainless
and brass.

As someone else pointed out, you needn't/shouldn't use the sealant on the
mating surfaces of a flare fitting.

Alistair