Thanks - the aircraft I'm thinking of are all AVTUR (JET-A / JETA-1) powered turbo-prop or jet, from 40yrs old to 5 yrs old!
The reason for asking is that I now occasionally have to operate above the max specified allowable OAT. The aircraft seem to perform well, but it's always nice to know what limit it is that you're exceeding before disregarding it! Engine cooling isn't a factor I don't believe and the same 49/50degC limit is spread across different types. The only common thing we could come up with is fuel (which does define the minimum temp we can use). The Shell website quotes a flashpoint of 38degC for JetA and JetA1but not a 'boiling' point.
Perhaps I'm just looking for an answer that isn't there!