I have been doing this so long that I have heard more don'ts than I can remember. I have also seen most of them turn out to be fable. I have always only retrofitted cars which have been flushed first. This really means that I only retrofitted cars that I was already going to flush; mostly cars getting new compressors or evaporators.
Originally there was great fear of mixing either oils or refrigerants. The consensus now states that the mineral oil just sits around as mentioned above. I have heard that PAG lubes better than ester, but I have heard that ester works better in non-flushed systems. The plain success of so many amateurs points to the lack of problems with the mixtures.
Bosch Master, ASE Master, L1
33 years MB technician