As someone who knows, dye from denim or any dark colour fabric, wil transfer onto your leather seats. You can buy certain cream protectors which help create a barrier but it is something you have to be aware of. I would also advise using a car sponge with the water rung out and wipe over your seats. This helps rehydrate the leather, with it being a natural hide and prevents the leather drying out and cracking. Your not cleaning with the sponge just adding moisture.