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IIRC about ten years ago 60/40 propane isobutane (R290) "replacement" refrigerants were being sold to DIYers for automobile A/C systems when it became illegal to sell the then used freon based refrigerants to unlicensed users.
There was quite a bit of talk then about the added danger of a propane explosion when the car was involved in a front end collision.
I've heard that those refigerants are banned in quite a few states.
Jeff