** I posted a similar answer in another area**
Cars prior to 1994 used R-12, and after 1994 all cars were equipped with R-134A. The reasoning for this is because R-12 refrigerant is a CFCs (chloroflourocarbons) which depletes the ozone layer, whereas the more modern R-134A does not contain CFCs.
If you mix r12 with r134 air conditioning system, it will cause a high pressure in your airconditioning system and eventually damage the compressor and the evaporator. R12 is only for r12 aircon and something with r134, it can't be mix.
Why would you want to even consider doing this? R12 is no longer in production and as such it cost almost 4 times as much as R134. But to answer your question, use R12 in a R134 system and you will destroy the compressor as the oil is not compatible with R134 oil.
R134a.
Should be R134
R12 134A pre 1996 is r12, after is r134
R134a
r 134
If if it a 1995 or older then it came with R12 installed. 1996 can be R12 or R134. 1997 up, it has R134/R134a. If you suspect it had R12 look for a label somewhere on the A/C unit signifying it has been converted. If you find it has R12 and is leaking and needs repairing, after the leak is repaired, convert it to R134a. R12 is far too expensive to use.
First of all, you don't use R12. Rather, R134 which has the oil included.
A 1995 Cutlass Supreme should use R12 freon. The fittings are different between R12 and R134 so that they can not be accidentally swapped.
This is the transition year. It can be R12 or R134. It is listed on the unit. The fact that R12 is no longer being produced makes it extremely expensive. Convert it to R134.
They should never be mixed.