As metal is impervious to solvents you can use pretty much whatever you have available. You might have to try a few to find the one that works best with the glue in the tag though. Kerosene is one option, white spirits another, methylated spirits a 3rd. Gasoline/lighter fluid might also work. Take some on a rag and rub the sticker, see what happens.
Be sure to use proper care when handling dangerous, flammable and environmentally detrimental chemicals.