wrote:
> ... why was only ester recommended
> for retrofits, at least until about 1995 (I haven't checked since then)?
>

Ester oil works with both R-12 and R-134. PAG is only for R-134. If the
oil isn't compatible with the refrigerant, it won't be carried thru the
system, will likely collect in the lowest point in the system, and the
compressor won't be lubricated.

I don't think PAG and ester will mix but I don't think they'll do
anything horrible if present in the same system (except clog it up if
there's too much oil). The best idea is to evacuate the system, fix the
leak, flush the old oil out & then add the correct amount of both oil &
refrigerant. Otherwise, you don't know for sure how much oil you have in
the system and too much can adversely affect the operation of the system.

Be careful fooling with A/C systems. It's real easy to screw them up and
cause lots of expensive damage. And if you don't know what you're doing
you can get hurt or worse. The A/C pros call those little cans with the
gauge 'death kits' for a reason (usually it's the death of the A/C
system, but if you hook them up wrong it could be yours).