Here's one of those opinions:

I have used a product called Enviro-safe (ES-12a) in a few vehicles, including our 1991 Volvo 240. These hydrocarbon blends have to be used in a leak-free system. I used it in an old Mercedes, and it was excellent. I got 38-degree air from the dash! I used it in my parents' Vanagon camper, and it worked great for a little while. It had a leak, and performance degraded rapidly. I put it in the '91 240, and it worked extremely well, as long as the car was moving. The problem with the '91 (not earlier 240's) is the condenser fan control. It works off of a pressure switch in the liquid line. When the refrigerant gets too hot, indicating that not enough heat is being removed by the condenser, the pressure goes up and turns on the fan. The Enviro-safe operates an a much lower pressure than R-12, and won't operate the switch. I took it out (the refrigerant) and recharged it with R-12. My neighbor had a stash of it in his garage, and sold me some cheap. I don't know what the '88 760 uses for condenser fan control. If it uses the pressure switch, maybe you could bypass it. If I couldn't get R-12 cheap (I can't anymore), I would hook the fan up to the compressor circuit with a relay. Enviro-safe is available from autorefrigerants.com, and they have a discussion regarding the legality of it.