Look at this site:

http://www.epa.gov/spdpublc/snap/refrigerants/hc-12a.html

From this site:
"May hydrocarbon refrigerants be used to replace CFC-12, commonly referred to as "FreonŽ ," in cars?
No. It is illegal to use hydrocarbon refrigerants like HC-12aŽ and DURACOOL 12aŽ as substitutes for CFC-12 in automobile or truck air conditioning under any circumstances."

Although it is mentioning other brand names, it is illegal to use any of the substitutes. Funny thing is, as I understand it, it is not illegal to buy or sell them, just to use them in your car.