Had a look at my own system just to check - Im sure it wont be a fluid that is topped up on yours it is gas so will need charging at a garage - you can get a top up over here for about £20.
used to be freon but now I think it is R134a a similar refrigerant gas but a bit more eco friendly
http://en.wikipedia.org/wiki/1,1,1,2-Tetrafluoroethane