Mayday is supposed to indicate a life threatening emergency and I agree with the observations of PACE as to a certain reticence to make a Mayday call. I have never done so but have made three Pan calls.
Once in a Hawk when the air con unit blew up (although we did't know what the problem was at the time, just a bang and some vibration).
Once in a Cub when the engine stopped as I was about to turn final (thought I could make the field when it happened - and I did)
Once in a PA 32 when the engine didn't seem to be putting out the normal power as I started to climb whilst coasting outbound across the North Sea and so diverted to Rotterdam. It was middle of winter and so called Rotterdam and asked for a diversion telling them we thought we might have a problem and wanted to land and just check that everything was OK. As we approached the Airport we began to get some slight rough running. As we flew downwind it appeared that there was some smoke in the cockpit and if I remember correctly I simply said 'Be advised we appear to have a bit of smoke' and at that stage they asked if I was declaring an emergency and I said yes we would call a pan. At the time it never struck me that there was enough of a problem to make a mayday call and I didn't think we would be unable to make a safe landing. As it was we landed on the runway to be met by a number of firetrucks racing up the runway towards us. It later transpired that a cylinder had blown and the smoke was caused by oil being dumped underneath the engine on to the exhaust system.
I don't know if the reticence was caused by British stiff upper lip or that in each case I didn't think that the situation had actually become life threatening.