I'm sure you are up on the detailing side of things, but I also used the gt11 until someone kindly pointed out it won't work on painted leather as it's sealed, so can't feed it or condition it.
However I recently switched to Dr leather Cleaner and Dr Leather dye block, mainly as we now have an X3 with very light leather, so between that and the M3, it's been amazing, the cleaner is much more efficient at removing any surface dirt and the dye block is amazing, it coats the surface to provide protection, yet both products do not in any way mess with the surface finish, it's stays Matt and non slidy.