And as has already been hinted, the sort of emergency this applies to is where somebody (you?) is definitely going to die if you don't. This is not for the normal worrying-about-regulations situation.
Not necessarily, as I said earlier, the advice is in the Flight Manual and it varies between types of engine.
The engines for the type I fly can use AVGAS Grades: 80, 100 and 100LL (MIL-G-5572) as an emergency fuel (i.e. should only be used whenever any of the authorized fuel grades is not available).
Restrictions: Do not use for longer than 150 hours during any period between engine overhauls.
150 hours is hardly a "do or die" situation but the running time obviously needs recording in the tech log if an emergency fuel is used.
(These engines can also use diesel fuel without penalty but with OAT restrictions).