Petroleum Dragon

Breeding
In the game it is impossible to directly breed the special dragons elements. There are several combinations that can result in a Petroleum dragon, note though that no combination is 100% sure. Below is a few examples that can result in a Petroleum dragon:

Venom Dragon + Mud Dragon

Mud Dragon + Dark Dragon

Neon Dragon + Nenufar Dragon

Damage Calculator
To explain, when attacking with Water, Petroleum deals double damage against Fire and Electric, medium damage against Water and Ice, and no damage against Plant. When attacking with Dark, Petroleum deals double damage against Water and Metal and half damage against Plant and Dark. When defending, Petroleum takes double damage from Fire and Dark, medium damage from Water and Plant and take no damage from Earth.