Freeze 12 is compatible with R12 oil. Therefore, theoretically, it is also compatible with R12 and contains 0% R134a. It may or may not contain flammable hydrocarbon gasses, I didn't see that specified in my quick look at their site.

However, although you probably COULD mix R12 and Freeze-12 and have it work fine, it is ILLEGAL. You could be fined by the EPA - especially if you go to a mechanic who attempts to work on your system and finds it wasnt pure R12 and not pure Freeze-12. He might report you for the inconvenience to him because he then has to recover the refrigerant and have it disposed of instead of recycled.

Once again, it MIGHT work - it PROBABLY would. But WHY would you do it? Remember: Illegal. EPA. Fines, fines, fines! Unless your system has a serious leak - and then it won't have much R12 left in it (if any) anyway - your BEST bet is to spend the $50-100 to go to a shop and have them inspect it and top it off with ACTUAL R12! You said it's only a bit low, so get the real deal. Freeze-12 is for people with leaky systems with no R12 left to worry about removing anyway.