Jmac

FWIW I spoke to a Cummins mechanic on this issue, here's what he said... You can use oil or fuel on the O ring...Replace O ring every time..Because the purpose of wetting it with fuel or oil is once the lid is back on( I tighten mine till it won't turn any more and have had no problems) the heat from the motor and the fact that the o ring is wet causes it to expand and seal the fuel system. B/C it's expanded is why it's so much harder to get off than on.

Jamie