PAG oil (Polyalkylene Glycol) refrigerant oil made for HFC (e.g. R-134a) systems CANNOT tolerate even minute amounts of chorides, such as from aluminum chloride "coatings" inside pipes that formed from the original R-12 charge or having over .5% or more of "chlorinated refrigerants" in the system.
Especially important is the fact that many A/C flush compounds contain these damaging chlorides. PAG oil is pretty fussy about its environment, if your system is "clean" then PAG is OK for you. There are several different viscosities available to suit wear charicteristics with compressors.
A newer form of PAG is Daphne, supposed to resist breakdown associated with chlorides.
Ester oil isn't so fussy, although I have seen where it will coagulate with the mineral oil used in R-12 systems if the system isn't flushed properly. Don't know what PAG with Ester will do, so if you have PAG already, might be best to stick with it...Cal

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