Synthetic benefits in differentials ans transmissions
When I lived in a climate that got genuinely cold in the winter, synthetic was a big benefit in differentials, transfer cases, and manual transmissions. These units didn't really warm up in cold temps, so mileage really did get better with synthetics.
Take a bottle of 90W gear oil and the synthetic version and put them in a 0 deg (F) freezer. Then take them out and slosh them around. The difference is (was, anyway) night and day.