In the case of replacing compressor and drier it is important to know WHY you're replacing the compressor. If it is just a shaft seal leaking, then you can get by with replacing it and adding a few ounces, but I personally would not do that. I would flush the entire system and put in the correct amount of oil. I say this without knowing the system. If everything else had been working well and it had never been recharged then just adding whatever volume you could get out of the original compressor would probably be okay.

In the case of replacing for a leaky shaft seal on a system with a history of leaks and problems, you should flush and put in the specified volume of oil.

In the case of replacing a compressor that has had internal failure the ENTIRE SYSTEM MUST BE THOROUGHLY FLUSHED! You should flush, flush, and flush again. If you don't ENSURE that all debris from the failed compressor is totally removed, then the act of replacing the first compressor will only be practice for doing it the second time.

This means breaking all connections and flushing thoroughly in both directions. Sometimes the serpentine condensors cannot be flushed enough and must be replaced.

Did I mention that you should thoroughly flush the entire system in the event of internal compressor failure?

Good luck,