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: