Where I live in the fall the gas stations switch to winter blend, which isn't as efficient. Add in colder [denser] air that your pushing your vehicle through increases drag on the vehicle. My work truck gets 13.5L/100km or 17.5 mpg in the summer and 18L/100km or 13mpg in the winter....
The 1HD-FT in my 80 runs seemingly very cool, so any extra heat is welcomed. On the hwy @ 120 kph [75mph] the needle rises above the lower marker, but a few minutes after exiting the hwy the temp drops. It runs strong, and I have no reason to think it's anything more than a diesel running...