Completed a preventative maintenance on a 13 year old Lennox unit. I changed customer supplied 16x20x1 filter, I took the split temperature and it was only 11. Due to the age of the system the customer just wanted to add refrigerant and see how far that will go. I have a 20 degree temperature difference. And a 8 degree subcooling the system is operating properly. Performed a amperage readings on the compressor 7.04 amps and condenser fan motor 1.0 amps system is operating properly properly at this point