Ever wonder why it's against the law? Propane is highly flammable and anything that breaks a line and can ignite it then you have a fireball under your hood! R134 uses a higher operating pressure than does R12. So if the time comes where you might need to do some work to the ac system it's wise to use a compressor that is made for 134a. If you have a 6 pack or hemi the choice is simple because you don't have the option.