Devices that assist in transferring liquid from commercially available fuel additive containers into capless fuel system intakes generally include an enlarged mouth portion and a narrower nozzle portion, with the mouth portion detachably engaging with an additive container, and the nozzle portion inserted into a capless fuel system intake, such that the capless fuel system's opening flap is held in the opened position. The mouth of the device includes a plurality of hinge points that permit circumferential enlargement of the aperture, thereby allowing the device to form a substantially leak-proof frictional engagement with the neck of fuel additive containers having different diameters. In this manner devices can be used with almost all additive containers, regardless of the type, size and/or manufacturer. The device may include at least one flexible region on the nozzle.