http://i38.photobucket.com/albums/e1...00octane_2.jpg
OK so here we have the engine life of a Merlin III at 12lb boost and in 1938 it was 10 hours and later engines it was 20 hours at 12lb boost.
20 hrs = 30 sorties flown exclusively at 12lb/3000rpm, or 80 sorties at 15min/sortie, which is already exceeding average aircraft life during the BofB.