I did my first complete leather upholstery valet/detail yesterday on a black M3 with tan leather interior. I qouted 2-3 hrs for the work. When the guy showed me the car he explained the leather had not been cleaned for, wait for it 11 years!. The backs seats of course were not too bad, but the front bolsters etc!!!!. I use my standard cleaner from my supplier and thier feed. Even using the cleaner neat and gentle scrubbing with a baby bath brush would not lift the dirt. As you know the folds on M3 seats are deep so you could see the original colour/difference. In the end I reluctantly used my general cleaner, testing first. Still would not lift the staining. Was i expecting too much after 11 years?