NOTHING should be used on fuel
Most boat builders use cheapy 'compression' fittings that are quite inferior to a standard flared union. Normal vibrations, etc. on a boat will cause the compression
nut/ferrule inside a compression
fitting to compress and deform the soft copper tube ... hence the leak and no matter how tight you run up the threads. Actually such compression fittings are for ONE TIGHTENING ONLY and should be replaced every time you loosen a fitting - the tube should be cut and a new compression ferrule installed.
Flared fittings (double flared is preferred) using stainless steel
tube is the SAFE way to make such a connection. Flare tools are cheap
- just takes some 'expertise' to use one.
FYI - copper is INCOMPATIBLE with fuel oil
- it accelerates the 'ageing' process of the oil
hope this helps.