Oilspill is a recessive, ultra rare marking that seems to function like the black version of Tobiano without the restrictions from the face and over the back of the neck or shoulders, or the black version of Overo without the restriction to the center of the sides of the body.

Oilspill must have concave shapes like Tobiano, but may be presented all over the body without limitation, up to 50% coverage. If Oilspill touches one or both eyes, the one or both eyes that were touched become red or orange in color, the sclera turns gray or black, and the pupil can optionally glow white. These traits are not inheritable to offspring who do not inherit homozygous Oilspill