Based it on my experience with my 1984 Isuzu P/U.
Back when it was new, I used 10W40 on summer and 10W30 on winter. If I used 10W40 on winter, the vehicle starts at a slower rate, like the battery does not have enough cracking power. Compared that to 10W30, the vehicle starts immediately. Also, I noticed on winter, when using 10W40 especially when engine is cold and drive it right away there is a drag like it is pulling something from behind. And also, I got terrible gas mileage at 10W40 on winter.
In fairness, if I used 10W40 on summer I get the same gas mileage as 10W30 on winter. This is based on my experience on this vehicle.
The oil I used on my other vehicles. my Escape is 5W20 & Jaguar is 0W20 are all year round. If I use XW30, I could get a different result but I "may" not feel the difference in engine performance but for gas mileage it surely will take a beating especially on winter. As the old saying goes, "Your mileage may vary"