R407c looks like the closest overall match to R22 refrigerant. It is very inexpensive. It has the same capacity. And most importantly, it matches the suction and discharge pressures well enough that metering changes aren't necessary. This video shows how to convert a R22 system to R407c. At least 20% polyolester oil is required in the system to have proper oil return to the compressor. Here is the 3 month update video.    • R407c Conversion 3 Month Update