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All mechanical air conditioning and refrigeration (not peltier coolers or absorption cooling) takes advantage of a liquid absorbing heat as it boils. To make a good air conditioner you need something that boils and condenses at "useable" temperatures and pressures. I've dealt with 3 different refrigerants in mobile A/C systems and what I've learned is they all work really well as long as you use the correct lubricant and charge amount. Starting with the most common they are R134A, R12, and R414A.

R134A is a hydrofluorocarbon compound that requires synthetic oil and a system that will absolutely NOT exceed 350 psi; 300 is safer. Badly reactive with water. Neither R134A nor the oils for use with it are completely non-toxic. Small molecules leak through perfectly good hoses unless they have a "liner" to contain it. It cools well, cost is moderate, and is easily available at any parts store or Wal-Mart. No license needed.

R12 is the original chlorofluorocarbon Freon. It mixes with mineral (petroleum) oils and will not degrade if the system temperature or pressure get high. R12 and its oils are basically non-toxic. Cools very well. Will not diffuse (seep out) through rubber hoses. No longer made or imported into USA; considered unavailable due to exorbitant tax.

R414A is a hydrochlorofluorocarbon/ hydrocarbon blended product. Cools exceptionally well; especially in systems with poorly designed condensers (small/ poor airflow). Works with mineral oil and is non-toxic. Does require barrier hoses with "liner." If your system leaks, the blend will separate and may cause compressor failure due to loss of oil circulation. Availability is limited and if not available you will have to live without A/C until you find it; or convert to R134A. More expensive than R134A and requires a license to purchase. Autofrost or McCool/ Chill-It are brands.

There are other "alternatives" that claim to be "drop-in R12 substitutes." These are actually propane, butane, R134A mixes, or even just straight R22. (R22 is an extremely high-pressure refrigerant used for sealed HVAC systems. It blows apart automotive hoses when the car sits with a hot engine.) Some are even worse. (MAPP gas) The flammable ones are deadly; ones that blow your hoses apart are worse than useless. Bottom line is this. If it doesn't require a license to purchase it is either FLAMMABLE or is an HFC-based (R134A) refrigerant with some "voodoo" additive to (hopefully) make it work (barely) with the original oil. I wouldn't use a flammable refrigerant for any amount of money; and if I want an HFC, I would just use the cheap, readily available R134A. It isn't that hard to flush the system and use the correct oil. If you know enough about A/C repair, go to www.epatest.com and get a license. If not, take your car to an A/C specialist before you het hurt or ruin your air conditioner!

Which one should you use? The choice depends on why you actually need freon to begin with. R414A is the refrigerant of choice if you have a complete, clean system that was working fine with R12; and for some reason you lost the R12. For example a hose blew, you hit something and broke the condenser, had to pull the engine out, etc. If the system was working well with R12, you can simply change the hoses for barrier-type hoses, evacuuate the system then charge in the R414A.

If there is any major trouble where you will have to flush the system out anyway, it's foolish not to go back with R134A-compatible oil. I have had the best luck with NAPA's Ester100 oil. Follow the label on the system for oil amount. If your system has been repaired properly, and is of a sound design to begin with, there will be little performance loss changing from R12 to R134A. This loss can be eliminated. Most Ford and GM domestic-built vehicles have plenty of condenser capacity and compressor displacement to work fine with R134A as-is. By installing a "variable oriface valve" (4-Seasons Smart VOV brand) in place of the fixed oriface tube; and using a compressor treatment additive called "Maxi-Cool" you can make the R134A work as well as, or better than the R12 did. And it will certainly work better than the old system did in the months before it finally died!

I've done engine swaps on both my cars and have built the A/C from the ground-up in one of them. Custom A/C info.