Looking in Subaru official retrofit guide, the process
Place the existing R12 in system outside the system.
Replace low and high hoses. These are costly. Not sure why this is needed nor do I know if the replacements are barrier hose.
Add 200ml of PAG 100
Evacuate and finish off with 30oz of R134a.
(rated capacity is 28-32oz R12)
The system holds 150ml MO.
By end, you'll end up with 350ml of oil total.
What's the consequence of having a roughly 4:3 mixture of PAG100:MO ?
OE retrofit guides were written a long time ago and many of them were never updated. The concept of dumping PAG in on top of the MO proved to be flawed. Each oil is miscible (dissolves into) with its compatible refrigerant, but is not with the other. The goal was that there would be enough of the correct oil being moved by the refrigerant to adequately return enough lubrication to the compressor.
Unfortunately the two oils inside an operating system will chemically mix to form an organic acid "goo" that is not miscible with the refrigerant and poor compressor lube return becomes a problem. POE oils eventually proved to be a little more successful (read not good enough) than PAG at mixing with the MO. This was never really adopted by the OE's, but the "retrofit kit" makers ran with it and the fist generation "death kit" was born.
The above is offered IMHO. I am not a chemist, just a student of A/C system contaminations and failures.
Flush the MO out first; would be the most current retrofit recommendation.
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