they are somewhat fragile ( at least on singers.. I have never dealt with a national ) and yes the fluids are the right track...fortunately there is not a lot of Contact area for them to stick. DO NOT use a screw driver. if you must "persuade" use something like an "orange stick" ( daughter and wife call them that it is a finger nail/cuticle pusher thing ) and/or a small rubber mallet... you are after vibration NOT force....you can soak it good and then using the wooden stick lightly "tap" in the finger nail slot to kind of move the slide back and forth till it loosens ..... time and patience... not force.......good luck