to fill the system, attach the can to the LOW side of the system with the a/c on. Slowly add freon and watch the sight glass. When the bubbles dissappear, there is enough freon now. There are r-12 to r134 conversions available. This would require all the old freon removed first. The system should be flushed with a special chemical treatment first. The air drier has to be changed out also. Then vacuum the system to -30mmHg and add the new 134 freon. There are many websites and write ups for this procedure on the web. I have done a few myself. The cold point for a r12 system is about 36 degrees versus about 42-44 degrees for a 134 system so yes, the r12 system is a colder system. Also, remember when changing the drier out, add .8oz of ND-8 oil to the compressor.