I use engine cleaner mixed 10:1 or and all Purpose Cleaner from the supermarket mixed around 5:1.
Remember that leather in car is spray painted the colour, so you need to think what you would use to clean paint with, just that it is on a soft surface.
The family wagon at 6 years old and 115k miles...
335i at 2 years old and 40k miles, but 40k miles in dark denim...
And when I sold my E350 estate at 2 years old....
I hate cleaning cars, so only get round to doing the interior every couple of months, sometimes 3 or 4 months, so I wan't something to get the job done properly and leave it looking factory fresh and matte.
I just bought a 1998 2,8i Z3 as a bit of a project, and a morning on the seats has taken them from saggy, shiny and hard to this...
All BMW leather for the last 25 years has been spray painted, and when you see it go hard, shiny and crack it is just the paint that is a problem.
The reason it goes shiny and hard is because you get grease and dead skin etc. forming a layer, just be aggressive and get it off with something capable of getting it off, too many of the dedicated 'leather products' just put another layer on top, will look good for a week or two but then that wears off or starts to shine itself.