The banjo joint at my oil cooler was leaking not long after i first got the bike. It soon began leaking badly and no amount of tightening would stop it. I just took a piece of #400 wet and dry sandpaper and laid it on a flat surface and ran the washer over it on each side till any marring was gone. reinstalled it and it hasn't leaked a drop since. Just don't run it over the paper with more pressure on one edge to avoid inconsistancy in thickness from one edge to the other.
