Hi, thanks you very much.
I will give you the french name of this oil and i will try to translate it word to word so don' be affraid. ^^
It is "huile de pied de boeuf" , "beef feet oil".
It smells a little. I use a brush to apply this oil on the back side of the leather, and a smooth piece of tshirt to apply it on the carved side.
Like this it penetrates slowly on the leather and you can adjust the hardness of your work and avoid the second effect that make your leather brown to dark brown.
The oil is just used to make the leather more smooth. Then i apply a common finish cream to make it shine.
Cheers.