To do the job correctly the garage will purge out all the old refrigerant and initially fill with a dye of some description to check for leaks. If all OK they will simply refill with refridgerant/oil and test the system.

The only moving part is the compressor. Corrosion and leaks are your main enemy. Its simple technology really and very reliable. But you must use it every few weeks, if you don;t the seals dry out and out leaks the refridgerant.