See our project on copper compression fitttings. It will not hurt to use PTFE or Boss white/green paste and yes, you can overtighten. Have a practice on a fitting and a piece of pipe first, it will save you worry in the long run.
i always use ptfe. you shouldnt put anything on the thread cos your just making it difficult to tighten. 4 or 5 wraps halfway around the olive and the pipe so that it fits snuggly into the fitting as this is where leaks can spring from. thats my pennys worth
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