Another problem with Freeze-12 is most A/C shops have Freon Identifiers to make sure the system is actually charged with either R12 or R134A. If impurities are found, most will either refues to service the system, or charge very high prices for the disposal of the contaminated freon. It can not be recycled in either a R12 or R134 equipment.
If one of these impurities is found, I will cost you 200 to 300 just to get the system cleaned out and then the repair and recharge.
My advice is DON'T let anyone put anything in your system except the real thing.
By the way, I get arguments about this, but just to convert a system, I evacuate, replace the dryer (not really necessary for the conversion, only a good practice), Hook up vacuum pump for 1 1/2 to 2 hours, turn off pump for 30 min, check for a leak of the vacuum and if all is OK turn the pump back on for 30 min, and charge with 134 using 85% of the R12 charge and add 2 oz of Ester oil and UV dye.
Did my 84 500 SEL this way 2 years ago, and get plenty of cold air hear NC. Did a friends 87 420 SEL 3 years ago with the same results. Have done many more cars and truck with equel sucess.
There are some GM and Fords which need the pressure cycling switch changed or adjusted, but any car with a real expansion valve doesn't need anything changed. I have had some Chrysler mini vans that cooled to good and I had to over charge them a little to raise the evaporator temperature.
I still believe, if the job is done correctly, almost any system will work just as well on R134 as it will on R12.
Just my $.01
84 500 SEL (307,xxx miles)