At times like this, you really gotta ask yourself,
Do I smoke?
Seriously, the car does carry 20 gals of flammable fuel, but it is liquid and has little risk of leaking into the passenger compartment since the gas lines run under the car and gas fumes are heavier than air.
I understand propane is an excellent refrigerant, but I smoke like a '63 chrysler.
Mostly in the car, where there is a potential for a leak inside the passenger compartment.
For this reason, I have eliminated propane-derived substitutes from my list of acceptable refrigerants.
One of the guys at work tried Freeze12, swore it was not flamable, and was pleased by the results. He sold the car 3 months later; so much for an unbiased long-term test.
I found to my shock that the 300E I recently bought has a working A/C with R12. Freeze12 is probably what I will try before I convert to R134 if it passes the bic test (light my bic, spray some of this stuff on it, if it does not burn, it goes in).
I would not worry too much about the long-term effects of alternative refigerant on the system since, when it craps out, your choices are
1) Rebuild it with that tank of R12 you stashed in the garage 10 years ago, or
2)Convert to R134.
You will replace the expansion valve and drier in any case, and the change in oils means you will probably be rebuilding/replacing the compressor within a year or so after change-over due to seal deterioriation. At least that is the way all the conversions I have driven have turned out.
May the force be with you...