Are they genuine leather? If so, you can clean it with Vodka (or White spirit) followed by massaging in some leather cream (leather shoe cream works just as well).
I'm honestly not joking about this! Perfected this technique while working on the beautiful Black leather interior of an old Datsun 240Z. Brings out the elegant original gloss and smells great!
If your interior is Faux/PVC leather, I've heard mild soap in warm water - applied with a toothbrush works.