[JRMichler]

I wish I knew exactly why the temperature has such a dramatic effect on my MPG. I do know that the air is about 30% more dense on a cold winter day compared to a hot summer day in Northern Wisconsin. When the air is 30% more dense, air drag increases 30%. Engine power also increases proportional to air density, and fuel consumption increased with engine power.

I have experimented with low viscosity and synthetic lubricants, and never saw a significant change. Now, I just put in whatever oil the manufacturer recommends.

I do know that my truck's rolling resistance increases a lot in cold weather. I suspect that much of that increase is from seal drag. Modern vehicles have many seals keeping oil and grease in, and dirt out. They all have friction. My hypothesis is that the seals shrink in cold weather, increasing drag. Someday I intend to borrow a thermal camera and look into it.