Freeze 12 is a propane based freon unless you want a new system stay away from it. To convert your car from 12 to r134a you need to drain oil from compressor and replace with PAG oil or a compatible oil. We use BG flouresent oil cause it is compatible with 12 and r134a , also will help spot leaks in future.
After you drain compressor pull vacuum on system for no less than 25 minutes , add 5 ounces of oil in low side hose and add 15% less charge to system . Say it takes 2.4 lbs of r12 , well I would charge it with about 1.9-2.0 pounds of r134a. Sometimes you have to start off with least amount and add while watching pressures and a thermometer in the dash. The only cars I do not recommend retrofitting are cars with weak compressors ( needles fluctuating, nippendensos are bad about this) , vehicles equipped with a-6 or old york compressors. The r-4's come out perfect for some reason and a good nippendenso will cool well also. Dont expect a miracle at idle in traffic though. Best nippendenso job yet was 46 degrees at idle in 100 degree weather. Rare occation down here . Well hope I helped good luck.
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck