The Birkenstock Milano Crosstown Leather Sandals guarantee maximum hold and offer stability with every movement thanks to the somewhat more compact upper with its two straps and wide ankle strap. The upper is also made of especially thick, oiled Nubuck leather with an open edged finish, and they are finished with a signature cork and suede insole. About Birkenstock Oiled Leather Oiled leather can be recognized by its untreated surface. Birkenstock purposely does not treat surface grains and other characteristic features. Birkenstock uses sturdy leather with a thickness of 2.8 to 3.2 millimeters as upper material.