Maybe wrong forum but will check opinions. Just noticed my OAR go from -.78 to -.90 with no changes other than season change. SE WI and do get winter blend fuel, it does take some time for the gas station tanks to flush summer blend same with my fuel tank. Only reasonable explanation?
Not complaining as it's a good change but I have consistently run -.76 or so all summer on a BCB Highlander tune with local 93 octane (same station even). KM

 

Colder weather will increase 93 octanes ability to resist knock. Same for 87 or 89.

 

Got the cold air bit but ambient has been fairly consistent mid day since August 45-65 degrees. Noticed this OAR change driving the other day at 60 degrees ambient. Gets cold and snows at nite, little early in the season. Just something that caught my eye driving. I have it displayed but it doesn't change much normally. Local fuel switches to oxygenated high volatility winter blend beginning of Oct.
I did just find this reference to winter oxygenated fuel, Gasoline Winter Oxygenates pretty much answers my curiosity. Just took some time for the OAR reading to adjust. KM