It is illegal to release r12 into the atmosphere... so if you want to be a lawful citizen take it to a shop. They will also remove any residue oil so it wont mix with the newer kind of oil. Some don't like to mix, and some say residue is ok. It is ok to handle r134a b/c it doesnt destroy the ozone layer, it is however potentially harmful/fatal to you (like r12) if inhaled in large dosages. If you ever take apart the A/C lines to the drier, you should replace the drier... it tends to smell bad once exposed to moisture in the air. The cans at stores usually have oil included with the refrigerant so you dont have to do the math. Just remember that you do need oil for the a/c system to last... there's a ratio, I dont remember what it is though. You should also replace the o-rings(or do a leakdown) if you suspect that you are losing refrigerant.