They say that air is the best thing, WD-40 eats the rubber. But I could be fooled, they also say that it has more chance of popping off the bead because it has a lubricating product inbetween the bead and the tire.....Best bet is too pull out the valve stem and air up that way because the valve stem only lets so much air come out, but with out the stem it lets all the air that can flow into the tire....