There's been lots of discussion on this topic; if there's doubt - there's no doubt:
"Pan, Pan, Pan" (or Pan-Pan, Pan-Pan, Pan-Pan) if low on fuel and
"Mayday, Mayday, Mayday" if
very low on fuel!
Nothing else will grab an ATCO's attention better.
And don't get suckered into the FAA's 'fuel emegency' side street either.