Look at the temperature-pressure curves of each material, as well as the critical temperature. Propane alone will give you too much head pressure. If your system were designed for propane, it would work not just good but great.

Hardware-store propane is pretty wet. Your grill doesn't care about a bit of water in the fuel.

Regardless of what the Duracool and Enviro-Safe makers say, moisture inside your AC system is indeed bad. And as for the instructions to let AIR into the system (or not pull a complete vacuum, which is the same thing), well I just think that's silly.