Diluted Simple Green is a good start for a product off the shelf. I have used it a few times with good results. You just have to be careful with the leather wrap on older cars so that you don't rub the color off. Once you have choosen a cleaner, do a small area to test the strenght of the cleaner before going on to the rest. I always go over the wheel with a wet rag to remove any cleaner residue before appying a leather conditioner when done. Some people may not due this step but its just me. I use Lexol for the final step.
Great question and I will be curious as to what the other pros use.