WAIT A MINUTE... you asked about ester and R12....
Freeze 12 is part R134a ISN't it ?
You wrote ' car has F12 ' since R12 was mentioned several times including the original question..
I assumed that was a typo... ??????
Here is the oil charge for Freeze 12... which says it will mix with the oils used for R12...
http://www.ussap.com/Freeze-12-AC-Oil-Charge-R12-Refrigerant-Replacement_p_1132.html
but more research is needed ......