About adding the oil. Since you're going to empty the system you can remove the larger hose connection at the firewall, or somewhere convenient, and pour the oil in the open connection. Inspect/change the O ring, put a dab of oil on it and replace/evac/charge.
You DO have an issue with how much oil still remains in your system. Leaks in an active system usually spirit the oil out with the freon. I suppose a little extra oil would be less disasterous than not enough...Cal