look up a product called dura cool. Essentially a hydro carbon based coolant ( like propane) , that allows you to use your existing system with the only up grade being an adapter to fill/drain the system. Cools about the same as r134, although not quite as cool. Used in semi-trailers. Good low cost to convert, no more volitile than R134 or R12, cheap to up grade, and the filler naturally combines with H2O therefor no real need to purge/scrub the system due to h2o contamination (i.e if you had a system that has not been working or may be considered "open to air" you just get the leak test done and fixed but you don't necessarily have to replace components due to moisture contamination), the leak test is not nasty to the environment, not nearly as much $$$ to refill as the product is less expensive, and you can future top if needed (they don't like it, as the installer makes money fixing the leak, and doesn't want the product leaking into the open air, but they can do it).
The bads are that the ac system is considered "dirty" at the point of filling/ changing, and you will have to replace most of the ac components (i.e. seals, filters,...) and have the sytem clean purged if you want to go back to R134. A sort of both feet in type of thing, however given what I have seen shops charge to convert from r12 to r134, it is about the same thing. It may not be quite as cold as r134, but it does work nicely!!!, and your system will run at a slightly higher pressure, on both sides of the compressor (not a biggie but just throwing it out there). I worked at a ford dealer and the dealer principle was looking into this as far as warranty concerns, but their used lot definitely used it to "top up" any used cars.
bob
