Going through my toolbox, stumbled onto something I've used in the past for thorough cleaning of leather seats - A 2" wide strip of the loop side of velcro (not the hook side!). The piece I have is some sort of strap, so it forms a loop which made it easier to hold onto.

May sound crazy, but I had an old M3 convertible with the dove grey seats that were just totaled. Tried everything I had on hand, but found this velcro strap and gave it a try. Worked great to get into the texture of the leather, but wasn't so harsh that it damaged anything. Obviously there are faster methods if you have the equipment, but this was a cheap/simple thing I had on hand.

Thought I'd share in case anyone has ever had trouble getting down into the texture. Since then I've tried the boars hair brush with a microfiber over it, but I haven't had seats that dirty since.