I believe the hydrocarbon (HC) refrigerants such as Duracool are a mix of propane and butane. The butane is needed to carry the lubricant. You can leave the old lubricant in there, or top off if any has leaked out.

I also believe you need to remove the old refrigerant before adding HC refrigerant. This means you'll have to pull a vacuum before charging HC refrigerant.

You can get Duracool refrigerant from duracool.com.