The Oil mutation is a limited mutation of Grow a Garden, requiring limited items or admin weather to access, as it is unavailable from the standard gameplay.
This mutation was initially characterized during the Kitchen Storm update, which was described by oil raining from the sky.
Before the update, players could choose between Boil and Oil to get a distinct weather.
Oil Mutation Multiplier Grow a Garden Wiki

Oil Mutation
Multiplier
x15
How to Get
During the Oil rain Weather Event.
Triggered by the Junkbot pet
Description
Mutated crop gives off black steam and drips, similar to the wet mutation effect.
The unique mutation is accessible through the Oil Rain event or by obtaining the Junkbot pet.
During the event, the island will transform yellow, and a sudden rain of oil will begin.
The mutation is indeed applied to the Junkbot pet; however, it is currently unobtainable.
Interestingly, the pet can be awarded as a special reward to the Albert Admin Abuse event participants.
The mutation will provide 15 times more than the sale value of the affected harvest.
You can notice your harvest mutated with oil if it emits a black steam effect.
Additionally, it will have a dripping effect, as seen in the wet mutation.
In simple words, you will notice oil dripping from your plant when your crop has this mutation.
During the event, if you find a light brown hue on your plants with continuous dripping oil, you can call it an oil-mutated crop.
The messy, oily appearance will make the affected crops easily distinguishable and collectible in the garden.
Getting Oil mutation is profitable and easier to access than other mutations, like Boil, as it is not limited to only through limited-time events.
The best thing is that the mutation isn’t limited only to any crop species.
Whether you have cheap or high-priced plants, you can get your plant oil mutated simply by waiting for the rain to occur in your garden.
You can act strategically by farming oil on low-value crops during the event and then using mutation-transfer pets to shift it onto rarer plants for resale.