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