If you're a quart low in your , there isn't enough motor oil to lubricate your engine properly. The extra friction causes drag that reduces fuel economy while you're driving around the area.
The same goes for dirty oil; it doesn't reduce friction properly. The result is you get to watch those numbers at your local gas pump rolling higher and higher.
The transmission also needs the proper amount of clean fluid to do its work. When it's in need of service, the transmission drags your fuel economy down.
So keep it clean and give yourself a fighting chance.