I replaced my fuel filler cap sal with the blue nitrile? seal and it does seal. But!!! when t was time to refuel (approx 2 wks) the cap was extremely hard to break loose. Much too hard for my wife to open. So I applied some silicone grease. That helped but only one cycle worth. Seems diesel fuel dissolves the grease. So I'm back to replacing the $18 blue gasket with a regular gasket.