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Usually fuel economy goes down during winter, denser air, cold temps and lower quality (lower energy density) winter blend fuel all contribute. Toyota has been pretty consistent with 75w90 gear oil in their car manuals from what I've seen, had a 2002 Corolla that took it, Hands manual was for 95ish to 2002 Corolla, all those years took it, 2007-2011 Yaris take it. They might have put in conventional and if you had synthetic in before you might see a slight decrease in FE, maybe a few percent, but not 3-4 mpg.
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