I use milk in my backloads. It's not the best I don't think(GI is somewhere around 30), but when you start adding other ingredients to it like chocolate syrup or dextrose, it can work well. I haven't done it yet, but I plan on having my PWO shake involve milk, whey, casein, leucine, and dextrose. With enough simple carbs, milk can definitely work. I would try to avoid raw milk early on in backloads though because there is significant fat content which will slow down the insulin spike.