The device includes a protective mechanism for preventing introduction of a gasoline-type nozzle into an inlet passageway of a fuel tank of a Diesel motor vehicle, the inlet passageway defining a central axis, the protective mechanism comprising two arms mounted at least partially within the inlet passageway, the arms being pivotable around a pivot axis substantially parallel to the central axis. A connector is mounted at least partially within the inlet passageway, the connector being movable in a plane transverse to the central axis, movement of the connector outwardly from an idle position relative to the central axis causing the arms to pivot from an idle position toward a free-flow position. The connector is configured so that insertion of a gasoline-type nozzle into the inlet passageway will not move the arms to the free-flow position, whereas insertion of a Diesel-type nozzle into the inlet passageway will move the arms to the free-flow position.