I just got a new pair of sandals yesterday. I usually get a 13 but this brand (Birkenstock) had me at 11. They hurt my feet. I got on their website and there was a good FAQ. The best part was about how the sandal facilitates a certain way of walking they call "rolling the toes", but it's really that old stalking technique, weight on back foot, front foot weightless, find a quiet place to set down the leading foot. So I have done this a lot when bowhunting deer and squirrels. But it was not my habitual gait because of having somewhere I must go.
I decided to buy into the new habit of walking like the sandals were trying to tell me. I wiped Fixin Wax on all the leather straps and set the sandals out in the very strong sunlight today. I don't know if it was the leather treatment or "rolling the toes" but now i'm in this brand new habit.