Usually you would use a special oil that is used to clean AC systems. You need to get it at a refrigeration supplier. Don't use any solvent that is not rated for air conditioning systems as it will do damage that you will not be able to repair short of total replacement. There is also a filter that is located inside the dryer that needs to be replaced. You should only touch it with pliers and soak it in refrigeration oil before reinstalling it. If you are not familiar with AC systems you may be better off taking the parts to a garage and having a qualified technician do the work for you.