Not sure why anyone needs to convert to R134. R12 is getting cheap these days as fewer cars are using it. I have 4 cans on Ebay for $80.

R134 is the only alternative to R12. Anything else will be a gamble and no shop would knowingly touch your system with a recovery machine if they knew you had put some sort of snake oil in it.

Before anyone says Freeze12.... Freeze12 is 80% R134 so it's not a direct drop in as it's 80% R134 so it won't mix with the oil so R134 is the only real alternative.

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With best regards

Al