The hide in question is coated leather, leather conditioners dont work on it. The older models (pre 2000) had uncoated leather which can be fed some products like Leatherique restorer. What has happened in this case is that the seat pad has sagged down.
BMW will not sell you the skin, they sell you the complete seat (some of them are actually made in Karachi, Pakistan by the manufacturers of Molty Foam)