Martin Berkhan, the guy that popularized the 16:8 "Leangains" approach puts milk in his coffee. From [0], "So, it’s a question of a dose-response effect. Can you have some milk in your coffee? Sure, I wouldn’t worry about it and I have it myself. Life would just be too damn boring with only black coffee, especially if you’re used to having some milk with it. How much milk/cream? I would put the limit at 50 kcal total used throughout the fast. That’s about 1 deciliter or 1/2 cup 2% milk."

https://leangains.com/questions-answers-5/