Why would you have to divert for fuel?
In the commercial world, we don't routinely carry additional fuel "just in case". It is entirely possible to have planned a flight perfectly adequately from a fuel point of view, arrive at your destination with your contingency, alternate and final reserve fuel intact, and through an unforeseen set of circumstances end up in a situation where you will land with less than final reserve fuel (30 minutes, or approximately 1200KG on an A320CEO). If you know that you will land with less than final reserve fuel, a MAYDAY must be declared.
I’m military through and through and if I had to divert for fuel (I have a couple of times) I would declare a PAN as a matter of course.
I can't comment on the military SOP in such a situation, but a PAN for fuel is no longer correct phraseology in the UK.