A lot of the tire shops are going to a nitrogen fill rather than compressed air. The theory is that nitrogen does not expand and contract with temp changes like compressed air does. For example blow up a balloon, tie it and put it in the freezer. Take it out an hour later and see that it has shrunk. Leave it out in the sun and it will expand. Supposedly nitrogen will not shrink or expand. I am not a chemist, just relaying what I have been told. Green valve stem caps indicate nitrogen filled tires. It is also compatible with normal compressed air.