[Mr.Fixit]

Octane requirements are based on the static compression ratio of the motor for the most part. Too little "octane" and the air : fuel mixture explodes prematurely. Mixing octanes of gas should result in a averaging of their octanes, given equal volumes. However, it can turn out slightly better that that because the octane boosting additives in the fuels are different respectively, and also added beyond the point of diminishing marginal returns. Mixing them brings the additives into concentrations with higher marginal knock-reducing capibiblity.


Furthermore, in winter, the air is more dense, hense more available oxegen to burn, so without carburation adjustements, you'll be too lean and knock. This extra gas is needed to absorb heat from the combustion chamber to prevent detonation. Higher octane gas in a slightly lower stoichiometric ratio won't knock and an be run at the proper A : F ratio.