Well, as far as my understanding goes, the only words ATC will need to hear, or indeed accept regarding fuel emergencies and to start reacting is "Mayday!"
There is no point in declaring a "Pan" as requesting a priority approach due low fuel means nothing to ATC. A Mayday is the only thing that will "get you ahead of the queue".
If you have not met you company SOP's for declaring a Mayday due to low fuel, by telling ATC that you will be declaring a mayday in ... minutes will atleast give them an idea on your status and will let them help if they can. If you declare a Mayday BEFORE you need to, be prepared to answer to the Minisitry.