Questions come up occasionally about adding oil to a system that may have lost some. The first thing you will need to know, is how much oil should be in the system from the start. The compressor manufacture should have documentation or, should be able to answer that question for you. The next thing to determine is how much oil has been lost? 

Once all of that has been sorted out, we choose our method of adding the oil. One way, is to pour the oil directly into the compressor via suction port at the compressor. Rather messy and difficult to do in most cases. The preferred and probably easiest method would be to use an oil injector tool. When using this tool, its advisable that there is some refrigerant pressure in the system  (3/4 charge would be sufficient). The discharge pressure of the system helps push the oil into the suction valve and immediately into the compressor. 

Once completed, adjust your refrigerant charge accordingly. The weigh-in method is best, followed by verifying superheat and subcooling. Below is an illustration from Yellow Jacket 69562. 

NOTE: This can be done on most HVACR equipment. If a high-side pressure port is not available, tank pressure from from the refrigerant cylinder will work as well.