The oil slick was a simple and effective utility item that enabled a droid to produce a thin film of oil across the path of the enemy. This resulted in slowing the forward progress of an enemy and allowed allies to retreat or attack using ranged weapons.
In 19 BBY, near the end of the Clone Wars, R2-D2 was captured by two Separatist B2 super battle droids aboard General Grievous's flagship, Invisible Hand. To elude capture, he released his oil slick, causing the battle droids to lose their footing. The astromech droid then activated his rocket boosters, igniting the oil and incinerating his captors.