Octan fuel is very important for engine performance. British used 100 octan fuel before BoB time in their Merlin III engines which allowed to got more boost preassure - 12 lbs instead 6 1/2 lbs ( for 87 octan fuel). These significantly improved low alt performacne (below FTH) of Spitfire MK 1 and Hurricane Mk1. ( I wonder if it would be correctly moddeled in Oleg BoB?).
So Merlin III engine ( Spit MK1 and Hurricane MK1) with 100 octan fuel changed boost preassure for engine ----> 9 lbs - Max. for climbing (normal rating)(1 hr. limit)
and 12 lbs - Max. for all-out level in special emergency ( climb and combat 5 min limit).
Spitfire MK V initialy appeard with Merlin 45 engine. Normal rating was + 9 lbs ( 1 hour). The question is if it was allowed from the begining emergency rating +12 lbs? I got manual for Spifire MK II with Merlin XII engine and there were +9 lbs - normal rating and +12 lbs -emergency rating. I got also manual for Spifire MK VA-B-C with MErlin 45,45M,45, 50,50M,55,55M engines. There is ratinging for engines:
+12 - take off,
+9 - normal ( max climbing 1 hr limit),
+16/+18 - emergency combat 5 min limit ( +18 is for M engines - low alt)
These manual is rather late version for all type of Spitfire MK V and engines. I dont have early manual only for Spitfire MK VA and VB with Merlin 45.
Still i think that Merlin 45 like any other Merlins with 100 octan fuel ( like MerlinIII, Merlin XII, Merlin XX or late Merlins 61,66, 70) has both normal rating ( +9) and emergency rating ( +12). Later in 1942 year emergency rating for Merlin 45 was rised for +16 lbs and for Merlin 45M ( crop supercharger - LF version) for +18 lbs rating. Shortly speaking Merlin 45 was standart Spit MKV engine, Merlin 46 was for higher alt supercharger settings and engines with "M" like Merlin 45M or 50M was with cropped supercharger for low alts.
If we speak about VVS Spitfires and any other lend lease western planes like Hurricanes, P-40, P-39 and others these all planes need for full performance 100 octan fuel. Probably Russian used only 87 octan fuel which casue derated performacne of these planes and lower boost preassures. Thats why Russian opionion about such planes wasnt positive. Some Russian planes have similar performacne with lower octan fuel.