Well, I'm biased too since I was remembering a R-12 to 134 conversion on an '89 Saab. So I did a little checking...

Mineral oil was original in that R-12 system.
Ester (also called POE) oil is used in the conversion kit.
Ester oil is also used in newer Saabs which have never had R-12 or mineral oil.
PAG oil isn't the same thing as POE...

By now Momobile is probably rolled up in a fetal position in the corner wondering what to do... So should he just get it evacuated & add the R-134a? Or does he really need to remove the compressor & drain it?