Are you sure that you know what kind of oil is in there now? If not, you should flush the entire system. I'd recommend using the newer DEC-PAG (46) oil in the system.

If you do flush the system, you should add oil to all of the parts. Figuring out how much oil remains in the compressor is the hardest part, and you may have to guess. You should try to get as much out as possible (after flushing) then add again. Do not use solvent or refrigerant to flush the compressor. Use new oil only. Flush all lines unless they contain mufflers. If you think any lines with mufflers need flushing, either have the muffler removed or replace the line.

If you don't flush the system, you have no idea how much oil remains in the system. You can guess, of course. Also you need to work fast, and not leave any unflushed lines open to the atmosphere, as the oil is hygroscopic and forms acid when mixed with water.

In addition to the dryer, you should remove the expansion valve to flush the evaporator, then install a new valve.