I am replacing a defective a/c compressor with a used one believed to be good. How do I introduce the pag oil? Is that done by the mechanic who will recharge the system, or is it done before the compressor is installed? TIAl

 

Many compressors come pre-filled with oil. Ask the supplier. It can be added when the system is open or by the mechanic charging it - but make sure he/she/it knows what you have or have not done. The oil gets distributed around the system by the refrigerant, so it is always hard to know how much to add (it would be less than the new spec).

If you are removing the old compressor, poor the old oil out into a container and then add a similar amount to the new compressor (if it's not pre-filled). Look up the correct type of oil and procure it in advance...