He advice I got on my new steering wheel leatherwas, cloth + water, don't get it soaked, don't use chemicals; but that is brand new leather which I'm hoping to maintain from scratch. Not sure if it applies to the heavily soiled.
The Dr Leather wipes are universally praised. Might be worth a PM to Ricey @ Revolve he did a great job on a load of leather recently. There's probably a thread or two in his trader section.