As the compressor has been exposed to the atmosphere for quite some time
already, the oil inside might be contaminated so I decided to pour out the oil
and put in some fresh mineral oil. It is easier to pour out the oil through the
discharge line, bringing the compressor upside down to drain off the oil.

57 Phase Change Basics 2: Setting Up a System

You will need to shake it now and then and let the oil slowly trickle out.
This will take some time and you can do it between intervals. After this, take
note of the amount of oil drained out and put back the same amount. I put the
fresh oil back through the suction line. Only put the oil back in when you are
ready to braze up the whole system.