R-12, the only refrigerant properly called Freon (it's a brand name), was the defacto standard for all automotive system and many small home appliances until 10 years ago. It's one of the highest efficiency, lowest pressure refrigerants available. When studies concluded that the chlorine based chemicals were hurting the earth's ozone layer, the governments of many countries made moves to phase out the production of CFCs. A replacement had to be found that performed as closely to R-12 as possible. After many submissions, the US and Japanese auto manufacturers settled on R-134a as the replacement in 1991. OEM systems were redesigned to accomodate the less efficient chemical which created higher system pressures, and did not carry petroleum based oils. In a system designed for it, the stuff works well, but the service industry was looking for a cheap "drop-in" replacement for R-12.

Hundreds of companies formulated "cocktails" of R-22, R-134a, Propane, Butane, Isobutate, and other stuff to get R-12 performance. Many of them have come to be accepted but the system must run ONLY one type of refrigerant. Each approved chemical has its own combination of service fittings to avoid intermixing.

The only two refrigerants widely used by any automaker was R-12 and, phased in during 1992 to 1995 model years, its replacement R-134a.

The scientific world has flip-flopped on whether this really was a contributing factor to global warming or not, but the deed has been done.