Have you flushed out the system with the right chemical? If you haven't, old oil will remain in the system, as well as pieces of the old compressor. You will be doing the same job again in a very short period of time.
A/C PAG oils typically have 3 viscosities, ISO 46, ISO 100, and ISO 150. I like how one poster recommeded going directly to a Ford dealer for the correct oil.
Do you know if the dealer can get you a new refrigerant system label? I would imagine the government would want such things to be easy to get, because refrigerants can cause environmental damage.