Hey I'm a HVAC mechanic and I would def recommend replacing the oil and leak testing if it hasn't been working for that long.
Oil in these systems absorbs moisture from the air and wrecks it. Also the oil helps keep the system sealed to prevent further leaks such as on the rubber seals on the shaft etc. so I would be replacing all the seals too which would have dried out and cracked - unless you want to be putting gas in the thing every year.
You should also run the air con at least every week or two to get the gas and oil circulating to stop the seals from drying out and cracking/leaking.

All that said, I have not done much work on car AC systems this is just what i have picked up from talking to my work mates etc over the last 10 years