ES-12a is propane. I've used it on a couple of cars and it's hit and miss.

On my old 1991 560SEL, it worked well. On my 88 560SEC, it's hit and miss (past 90 degrees it sucks).
On my latest 560SEC purchase, which was converted to R134, it blew cool but not cold. I sucked it out and installed ES-12a, it was not cold at all. So I sucked it out and put R12 in and it blew cold.

So on this one car which I suspect has a marginally working A/C system
R134 is cool
ES-12a is ineffective
R12 is cold.

I just charged a buddy's 300SD this weekend and it worked quite well.

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With best regards

Al