I just took the Virginia saltwater stickers dating back to 1991 off of my 18 this weekend. With my patience, I was able to peel all of the stickers off leaving only the adhesive residue. Used WD-40 first to soften it up, then, used acetone to remove the balance of the adhesive and the wd-40 residue. Slick as a whistle.
Your results may depend on the type of adhesive they used. We all know some stickers come off easier than others.