the answer for that depends on the manufacturer of the compressor. Some compressors come with oil others come empty. The ones that come empty say to remove the oil drain plug and drain the original compressor. Add the same amount recovered from the old one into the new replacement. Compressors that come filled should be drained and then add the amount that came out of the old unit. Sometimes a new system, (new everything) may come preloaded with all the oil for the system in the system parts. The oil that the system needs will be spread all through the system. If less than one ounce is recovered from the old compressor, add two ounces to the compressor.
after installing the compressor into the system, rotate the compressor by hand around ten times to circulate the oil through the compressor. This will get the oil through the compressor instead of having the compressor start up with the engine spinning it and damage the compressor internally by trying to compress the oil in the cylinder.
the OE compressors were: Harrison compressor: takes PAG 150 oil Denso compressor: PAG 46 oil
Check for lables that should be in the hood area for AC fluids capacities
you can put the oil in after the compressor is installed but it's better to do it before it's installed. It depends on how close the low side port is to the compressor. If the low side port is right next to the compressor, then when you add the oil it will be drawn into the compressor right away. If the port is a long way away from the compressor then the compressor will run dry for a while before the oil will get to it and there could be internal damage. The damage may not disable it but it it will shorten the life of the compressor.
If you add oil with a new compressor you would add it to the low side port. You have to evacuate the system of air then add the oil to the low side. Let the vacuum pull the oil in. Then you have to be vary careful because the oil will be sucked into the compressor and the oil can't compress. So you have to turn the compressor by hand until the oil circulates through the compressor. If you start the engine and turn on the compressor it will spin at what ever the engine rpm is and when the oil hits it it may stop the compressor. (or drag it way down)
If the compressor manifold is on the top and the compressor is accessible, remove the manifold and pour the oil into the compressor directly while turning it by hand. Use a small funnel will help. pour a little then turn it until it all gets run through. Then bolt the manifold back up and it should be okay. Then when you go to evacuate the system evacuate from the high side. Keep the low side gauge closed. This way you don't suck the oil out that you just added. Then when you charge it you go into the low side to push the oil through.