The oils are definately not compatible. R-13a uses PAG (Polyalkylene Glycol) or ester oil. R-12 uses a mineral based oil. I think in your case you would have to evacuate and flush the entire system before reintroducing R-12 in it. But I'm sure others on the list much more knowledable than I can give you more details.

Good luck.

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J.B. Hebert

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