When it comes to leather we trust Colourlock. They are the leather experts and out-class any detailing company when it comes to leather products. They have a huge and thorough knowledge base already in place and the company responds well to emails asking for help if you run into a rare, specific issue such as dealing with old or damaged leather.
Additional Resources
How to clean and maintain leather
How to clean and maintain Alcantara
How to clean leather (YouTube, German with subtitles)
How to protect leather (YouTube, German with subtitles)