Bit of an old thread, but I did watch a video last night related to this.
Leather cleaner or diluted APC (I use 10:1 for interiors) to start. If that doesn't work, break out the magic eraser. Follow it with a good leather protectant, as it will sand away the top layer of clearcoat and make it susceptible to cracking and more dirt.
Professional detailer had to resort to magic eraser on a 1991 Bently Turbo R with cream leather interior. Seats, door cards, console, dash... all cream leather, all filthy after 28 years. He spent 3 hours cleaning it with a Zymol leather cleaner and magic eraser and soft-bristle leather brush. That was another trick, don't just use plain water, mix in a good leather cleaner.
LUNA
2002 Chrysler 300M Special - Deep Sapphire Blue/Storm Gray Metallic - 142,000 miles
The fleet
2016 Jeep Wrangler Unlimited Sahara | 2020 Dodge Durango R/T AWD | 1929 Ford Model A Tudor