6.9 IDI thread seal for top of injector?

Im taking a moment thank everybody who has replied to my questions. Every response is appreciated.
Existing fuel supply line to new injector leaks at compression fitting. Concerned about stripping threads so tight, still leaking. Advice?

 

are you sure it is a supply line leaking and not a return line cap?

 

Yes. I can see fuel bubbles along with seepage if fuel coming out from under the nut on supply line

 

Yes I can see bubbles along with fuel seepage coming out from under the nut of the compression fitting at the top of the injector

 

Tony, the bottom of that nut is the top of your return cap. I wonder if the upper oring in that cap is damaged and letting it leak out the top of the return cap?

 

I agree, that is is most likely a return line issue. However, take a close look at the sealing surface (flare) of the injection line. It is possible that the flare is damaged, misshaped or cracked. Do NOT use thread sealant, as the threads aren't meant to seal anything.