On a specific trip of 56 miles I take frequently, I get a consistent 33-34 MPG in the summer, not counting days with rain, fog, etc.
And with the temp at 65º yesterday, it dropped to 31. I expect it to drop to 28-29 when the temp gets lower.
No question, just a comment. But I do wonder why that is not being addressed by the manufacturers. Surely a better lubricant can be found. That’s a lot of gasoline wasted.
Subaru forester 2015, basic engine.
I wouldn’t jump to conclusions on just one trip. And you’re also making a wild assumption that it’s a lubricant issue. There could be several other factors.
Coming back from South Dakota I had about a 30 mph head wind and my mileage dropped to about 25. This time of year, it gets pretty windy and that can have a pretty dramatic affect on mileage. I also used to notice that on my 200 mile trips to school, I would get better mileage one way than the other. I really don’t know what the reason was, maybe wind, change in elevation, who knows? I just don’t pay much attention to these minor issues anymore.
Cold air is more dense than hot air. More dense means more oxygen per unit of air. This means the engine computer will inject more gas per cycle to maintain the proper fuel/air ratio. This makes you use more gas, but also makes your car a little more powerful.
Coupled with cold weather is generally a decrease in humidity. That’s why we start itching and use a lot of chapstik in the wintertime. Decreased humidity means less water in the air, which again means that a given unit of air will have more oxygen in it, because it’s not being crowded out by water.
There would also be an aerodynamic effect – dense air is harder to push through than not-dense air, which is why airplanes can go much faster when they’re up high than when they’re at ground level. Admittedly, the aerodynamic effect on a car tooling along at 60mph is much less than the aerodynamic effect on an airplane cruising at over 600mph, but it is still there.
Now, lubricants do have an effect, but not for very long. Once your engine warms up, it’s really hot in there whether it’s 80 or 40 below outside. It’s plenty warm enough for the oil to flow as expected.
Your transmission will gain residual heat from the engine as well as make heat of its own when its components spin, so the transmission fluid/oil will also warm up to flow normally.
Unless you make a lot of short trips that don’t allow your fluids to come up to temperature, you won’t have a huge effect from higher viscosity in your lubricants.
But, all of these little things (and a few more factors to boot - for example, pushing through snow is more work than driving on clear, dry pavement, so if you do much of that your mileage will drop) add up to provide for the drop in mileage.
Double check tire pressure first thing in the morning - it’ll drop with the temp.
I’ve lost about 10% in mpgs during the winter (southern Ohio) on every car I’ve owned, even in the days with the exhaust manifold stoves that preheated intake air. It appears to be “the nature of the beast.”
Does your area use winter/summer gasoline formulations? They might have switched to the winter formula. My gas mileage drops a little when the winter gas arrives.
I have a history over last winter of MPG dropping with temp. And it’s the same gas that gave me 34 MPG on the previous trip a few days before when it was warmer.
Remember I have 3 differentials and 8 CV joints.
In dense air, the throttle has to be closed more than in non dense air and so the engine consumes more of its gross power output in pumping losses when making the same net power.
Humidity has a minor effect. Even when the relative humidity approaches 100%, the absolute humidity is seldom more than a percent or two.
It’s not just high temperatures that are gas mileage friendly, driving in regions of high elevation also gives you better gas mileage. Many people report surprisingly good mpg when traveling through the high country, New Mexico, Colorado, Utah, etc.
Another factor in play is that your engine needs longer to get to operating temperature. Cold engines burn more gas, even if modern fuel injection make cold engines very driveable. In the summer, your engine is already pretty warm when you start it and so it has a head start in warming up.
Winter gas blends tend to have a lot of light hydrocarbons blended into them, butane for example. This make cars easier to start in sub zero temperatures but also reduces the fuel’s density and the number of BTU’s per gallon.
Manufacturers HAVE taken cold weather into account. They’ve richened the mix. 
Remember that combustion requires not only hydrocarbon molecules and oxygen atoms, but also heat. Since the air being drawn into the engine contains less heat, more fuel is required to maintain sufficient power to operate the vehicle. In fact, your car contains an oxygen sensor in its exhaust, whose signal to the computer helps to keep the fuel mix as lean as possible to keep emissions down, but your computer has to ignore the oxygen sensor’s signal when your engine is cold to have enough gasoline in the mix to keep the engine running.
Besides that, there’s all the other factors mentioned; tire pressure, air density, etc. etc.
The ultimate solution? An EV. And they’ll be ubiquitous soon. 
Great point!
Almost everything I mentioned has a minor effect. But stack enough minor effects on top of each other and you start to get something noticeable.
This is true, although a lot of us in the frozen north have heated garages, so our engines start out at a nice 60 degrees or so, yet our mileage still suffers come winter.
Another great point!
EVs aren’t immune to winter blues either. My Tesla-driving friend reports that in really cold weather his range can suffer by as much as 50%. Fortunately, when your range starts at 200-300 miles, cutting it by half still usually means you can get to work and back, but the batteries are definitely less happy in the cold, in addition to having to run the heater which saps energy.
The model airplane guys sometimes use a battery oven in cold weather. Also some of the EV dragster guys use battery heaters to get the most power out of a battery for a run down the drag strip. The guy who runs the KillaCycle elecric drag bike reportedly likes to preheat his batteries to around 167 degrees F (75 degrees C) before a run.