The sell leather cleaners and conditioners at the parts store. You sould be conditioning it atleast ever 4-5 months or more to keep it looking new and help againt the dry-cracks. The leather they used in the Mustangs suck, Im glad I have cloth seats.
When I still HAD grey leather this is what I did. (Attempt at your own risk). I had a soft nail brush and windex....(WITHOUT AMMONIA!). When you spray it on it will leave LIGHT colored spots but that is normal. Use the SOFT nail brush to get the grime out of the cracks. Wipe with a clean cloth afterwards. Once dry...treat it with leather conditioner. Try it on a spot on the leather that people WON'T see first to make sure it won't fuck'em up...like between the seat and console. Sounds janky I know but it worked like a champ for me.