I am curious between the difference between synthetic and human gray in units. I currently have 20-40% gray in mine, and specifically asked for synthetic due to using BackToNatural after the unit begins to fade to extend the life. Synthetic gray won't color, but it doesn't behave like human either. What are the benefits/drawbacks of using natural gray in units that need at least 40-50% and up? Does natural gray color that much when using BackToNatural?