I had my A/C quit working over the winter so I did a R12 to R134A conversion. I already had the fill kit for the 134 so I bought the fittings to convert the high and low service ports to the 134. All went well but I was told to make sure that I removed all of the R12 from the system before I filled it or I could damage the compressor. I did not add any oil at the time I filled it and then I was uncertain about adding any later because I didn't want to overfill the system. Well to make a long story short, my A/C worked great for about 3 weeks and then one day it quit and the A/C light was blinking. I open my hood and turned on the A/C and sparks jumped from the compressor clutch. Now my compressor is locked up and I will need to replace it. So my advice is that if you do it yourself, evacuate the system and add oil before you add the 134.