In your ketucari's genotype, oil is denoted by the letters "nO" (heterozygous) or "OO" (homozygous).
In its heterozygous form, oil has a pass rate of 35%. Homozygous oil has a 60% pass rate.
Color and Shape
Oil is always a dark color (black or close to it) with an iridescent overlay to mimic the appearance of oil. Though the edges must be hard, they may be spotty and irregular, like on Sirahn.
Here is a useful tutorial on how to get the iridescent effect:
Since oil can appear anywhere on the body, there is no minimum range diagram for it. It must cover enough of the body that it is immediately recognizable, but not more than 50%.
Interaction with Other Markings
All markings can be layered under or over oil.