The original question behind this post is what we think about using propane as a refrigerant.
I repeat that this is not something I want to do.
I just found out in the last two weeks to my surprise that the A/C on the 300E I recently bought works, and found out last weekend that it has no screw-on R134 fittings, which I assume means it is still charged with R12. I have not yet tested that theory.
I bought this car with the assumption that I would be either converting or rebuilding the A/C system before June.
Since it now appears possible to get another season out of the existing system, I choose Freeze12 as an alternative if the system needs a booster shot.
The day my shirt gets damp, the system gets converted to R134 the following weekend. This will be the second one I have done, and for a stretch goal, I plan to either rebuild or kill this compressor. (whichever comes first)
This is simply a personal battle-plan for dealing with unexpected fortuitous happenstance.