my AC system in legend had a leak and it still does, i'm not sure if i will fix ti since the hose that is leaking costs 250$:( but i vacoomed the system and got it with artificial r12 with die that i got off ebay and it really helped to find a leak, why is it harmful and to what? rubber seals and rubber hoses? the nice part is that you dont have to change oil in the compressor if you use artificial r12.:) saved some trouble for me:) otherwise as far as i know you have to suck old mineral oil out and put the one designed for r134, if you mix r134 with r12 oil it will foam and destroy the system.