I talked to a mechanic once, maybe he is out of his mind... But he had a specialization in AC. He said I could just put in the 134a in on top of the R12. What he said was, the 134a gas won't carry the R12 oil. So what happens is the oil from the r12 sits around in the compressor oiling it. What he also said I needed to do was buy R134a charges with the ester oil in the can... Or buy a seperate ester oil charge. This is what I did and it's worked for months, even through summer in florida, where I am and it's still kickin like brand new. No seals, no draining, none of that.