if I had an M3, royal purple products wouldn't come anywhere near it..
really - how often do you have to change the diff fluid? maybe 3-4 times over the life of the car? I feel the same way with coolant. Yeah, it's an extra $20 for the BMW stuff - but that's spread out over two years. I've seen the damage from using cheap fluids and it's just not worth it. BMW stuff is formulated exactly for your car, you don't have to guess or hope it will work.