As a fellow Hot Country Texan, I had to chime in on this one. My auto
A/C man is a personal friend of 25 years, not a German shop. In my quest
for R-12 replacement he showed me the list of all substitutes including
FR-12 and he told me all apparently work well. But the downside is that
nearly all of them require guages or special equipment that the standard
auto A/C shop will not have. jThis would mainly be the recovery
equipment they have. In the servicing of your unit, with thier recovery
unit they are not allowed by law to co-mingle any of the diff
substitutes in their recovery equpipment. 
All refrigerants -must- be recovered if a system is being worked on. And
theoretically (legally) they must have a separate recovery unit for each
refrigerant they service. Secondarily, they all wiegh in at different
wieghts, and the shop must have a different unit for wieghing the
refrigerant when they put it back in (in case the system is low and they
have to add some.)
My friends shop has settled on only converting to RF-134a and are
apparently having very good success. This forced them to only purchase
one additional set of "tools" to maintain these systems. What they have
learned (not sure where) is that the RF-134a does indeed run at a higher
pressure that R-12 and they were instructed to fill the system w/20%
less (by volume or wieght: I'm not sure which) and that this releives
the pressure on the system (compressor) and cools almost exactly as well
as R-12.
There's what I've heard. My system is still R-12 until I need a bunch,
then I will go RF-134a for the above detailed reasons.
My nickels worth.
Mark B. Magee
87GL 83K
Kemah TX USA