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.
It depends. If the refrigerant was lost with a slow leak, then most of the oil may still be in the system ("most" because you would have lost some changing the components out). If the leak was larger, some oil may have escaped.
Without fully draining and recharging the system, it's going to be a guess.
That said, unless there's evidence of refrigerant oil that leaked out, I normally will assume that the oil is still in the system and just top it off based on the components removed. Sometimes it's not practical to fully drain a system, especially an older one where a lot of connections would need to be disturbed. If you don't add enough oil, the system is obviously at risk. If you add too much within a reasonable amount the oil normally just collects in the accumulator. However, any extra oil displaces usable refrigerant, so it would reduce the cooling capacity of the system.
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.