I've never heard of R-18. Are you sure you read the label correctly? If so, many of us, including myself, would like to know more about what you're trying to service.

From what very little, and possibly inaccurate information I could find, was that R-18 might be used as a refrigerant for cryogenic processes.

Found this, What is cryogenic refrigerant 2 slides of information, again I don't know if it's accurate, but it mentions salt water progressing through freon to become r18.