Lamp oil is made from the oil of a certain plant, a small orange-tinted ball that grows in the shadow of large trees.
At first seemingly innocuous, the so-called Firebrand Flower shows its true colors during hot summers. At first, the pores on the round, cactoid body of the plant opens, allowing the oil to evaporate. This clear liquid clings heavily to all the plant life around, at first dehydrating them, and then self-igniting in the heat of the sun.
When a colony of the plants are together, a great forest fire generally ensues. And afterwards, the second form of the Firebrand grows in the ashes, the flower itself. It has large jagged orange leaves, which mildly relieves hunger if chewed.
The use in harvesting the bulbs before they can combust is twofold. The prevention of fires is important to both the lord of the lands, and its residents, for wealth and life respectively. And the oil is useful, if dangerous. It’s easily ignited, and burns brightly and hotly. It’s often used by adventurers for use both as a light source and as a weapon.
(Trying to justifying the D&D-ism of lamp oil somehow being napalm)