I would advise against using compressed air for a leak checking media. There is more in compressed air than just moisture. This does not however mean that you cannot use air. When you introduce the air if it is from a oil type air compressor it will always have a trace amount of oil in the air. The oil that is in your compressor will gum up when it comes in contact with the mineral or poe oils, this will cause problems with the refrigerant metering devices. An oiless compressor would be a safer bet if you are going to use air.