I have a 2003 Citroen c5 which I bought last year, the A/c system has been empty for refrigerant since before I bought it. I have purchased the necessary equipment to vacuum, and recharge the system. in the manual, it says that the system takes 135ml of compressor oil. I assume most of this is still in the compressor. how much oil should be added when adding refrigerant? I am also changing the receiver drier.
You should evacuate the system, then establish a depression on it and leave it to find any leaks.
If there are no leaks then the manufacturer will state the amount of refrigerant and oil to be added.
You stated in your question that it requires 135ml, so remove any oil still in the system then add that amount.
The systems used by both my garages (one in UK, the other in CH) will control all parameters, oil, refrigerant and test accordingly.