As an FYI the Propylene glycol they have at Tractor Supply doesn't work very well, even though it's U.S.P. It's very oily and WILL NOT mix with water, VG or flavorings; anything added (except alcohol) will just bead up and separate no matter how much shaking and yelling you do. Anyone know the difference between U.S.P. grade and food grade? Where do you guys go to buy PG that will play well with others?