I hate to be the one to bring in bad news, but most leather care products you'll find locally are actually bad for your leather as they leave residue that attracts dirt and causes the urethane finish on your OEM leather to wear out faster.
The only two brands I trust are Leatherique and Leather Master (...well... and LTT for you guys in Europe). Use their products as directed and you'll be fine. Your leather has a urethane coating on it, so there's nothing to "treat" just as you don't treat your paint; you protect the clear-coat which is protecting the paint. You simply need to keep the finish clean and free of dirt and debris that will wear out the coating.
I don't trust the BMW branded leather care products simply because I know nothing about them. How are they formulated and for what purposes? Are they being made by leather companies, or are they made to help dealerships sell products that customer's are interested in buying?
For the cost of purchasing multiple cheap products that don't work, you could easily have top-notch products that are professionally proven and will work perfectly. Make the right choice the first time and your leather will look better for longer.
5 and a half year old S2000 seats with 70k miles traveled on them after treated with Leatherique:
2001 Lincoln Continental half way done with a Leatherique treatment:
Red leather seat of a BMW 135i before cleaning:
After Leather Master Strong Cleaner and Protection Cream:
Leather in this '97 NSX cleaned and protected with Leather Master Strong Cleaner + Protection Cream, other hard surfaces cleaned and protected with 1Z Cockpit Premium:
The right products and technique are well worth the investment and time.