For light spills always use distilled water and a clean cloth. Wipe the spill in one direction. Application of a leather conditioner is useful in repelling stains. Most stains will disappear with age. Not all stains will come out.
Excessive exposure can fade and dry out leather upholstery.
If a leather has been conditioned and maintained it should be more resistant to scratches. Most light scratches can be massaged lightly with clean fingers. For deeper scratches there is little that can be done, remember this is a skin that no longer has healing capabilities. When using a pen ink, allow the mark to dry then use distilled water and the cloth. A suede eraser can be used on a dry surface if the stain persists.