Couldn't agree more 'A2' but it's often the fear factor of sending out a "Oh Bugga" call of any denomination as that implies a level of perhaps....."****e I've stuffed this up"... in some cases followed by the usual paper work, tea & bickies in a "Just Culture" environment...cough cough cough!
There's Med 1 used when it's obvious that urgent passage is required, you get priority except where an A/C is in danger (or Kev07 is inbound
)Med 2 less urgent usually reserved for the transportation or returning of medical personal.
Pan & Mayday, both achieve/receive the appropriate responses when used in the context of the included message IE: PANx2 PANx2 PANx2, Eng failure etc. Or MDx2MDx2MDx2 Engine fire etc.
If in doubt use MAYDAY, you can always downgrade to PAN if you get the fire out for Eg or the threat is no longer of imminent danger.
As for low fuel & a level of urgency needs to be conveyed to all out there a PAN absolutely should be used I believe with a follow up Mayday when it becomes critical.
No pilot likes to use these terminologies we all know that but going by the book at the very least covers a certain section of yr ass that's hangin' out there !
Wmk2