R134a does react with mineral oil, which is the typical lubricant used in older systems like R12 and R22 systems. With R134a, they switched to newer lubricants such as POE. This is why you need to flush the compressor and whole system when you retrofit a R12 system to R134a. You have to remove all traces of old mineral oil as well as ensure that no moisture has crept back in during the retrofit.
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