Wouldn't boss green/white be the best option. That would fill any small gaps but still allow the olive to compress as it would just squeeze it out the way. Anything solid like ptfe tape would make a gap and it's the olive that should be compressing.
In an ideal world you wouldn't need anything. But if the pipe had an old olive you removed, the capnut didn't come with the fitting, you can't get the recommended amount of turn on the nut before things get difficult (even if you DO know what the book says), or the whole fitting quality is poor, then some sort of compound makes a sucessful joint more likely.
Liquid PTFE's don't set hard (that I've seen) whereas oil based things like Boss White do eventually. PTFE tape on the olive seems to work - it squashes to nothing. Jet Blue seems to be better than many goo's.
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