I use 4 auto as soon as there’s a possibility of slick roads. It doesn’t affect gas mileage one bit and there’s no downside to doing so.
On out way to LOTW on Monday, I was side by side with another F150 taking off from a stoplight when the temp was -10. I could see that the lane the other truck was in had tire tracks that looked almost wet and I assumed it was glare ice. So I let him pull ahead of me by a 1/2 car length just in case and sure enough as soon as he started accelerating a little harder he spun in my direction and across my lane into oncoming traffic. Luckily he didn’t hit anything. 4A in that instance would’ve completely avoided the situation altogether.
I only use 4H now when I’m in deep snow or mud.