From what I've read, HC blends work well in R12 systems, although not quite as well at idle. Better at idle than 134a, I'm sure.

The problem with HC blends is that it is illegal to convert directly from R12 to HC. Since mineral oil is miscible in HC liquid, it would be ideal to extract the R12 and charge right up with HCs. It would be better for the environment than 134a. But it's illegal and the penalties are quite severe.