Oil is a resource that can be found in the Underground City. You can extract Oil using the Oil Pump found at the middle below the Pyramid in the Underground City. It can also be obtained from Treasure Chests or as a reward from scientific excavation mission. For the exact amount of obtained Oil look at the table here.
Oil is used as a material to upgrade the Drill starting at level 12 and to upgrade weapons in the Underground City. Unlike other materials Oil can't be sold.
Oil
