Kestas offered a very good and accurate reply. I only want to stress how important it is to flush the system, followed by flushing the system, then follow up by flushing the system. On top of that operation it would be good to use a suction side filter that you can get from your auto ac parts supply.

If you are changing to r134, the flushing will remove all the mineral oil and make way for PAG or Ester. If you do not flush (did I mention that if you had a failed ocmpressor you should flush?) and are changing to 134, Ester will work better. If this is a Nippondenso compressor, PAG 46 is preferrable, but ONLY if you have thoroughly flushed the old oil out. Replacing the filter drier should be considered absolutely mandatory with the system being opened AND a new compressor installed.

This is not an accumulator system, this means that the filter drier is after the condensor and before the evaporator. If you had a failed compressor there will be lots of junk in the evaporator with no filter between it and the compressor. Without thorough flush and suction side filter, chance of compressor failure is high.

Good luck,