Harvested from either wells or oil factory and is needed for most if not all the machinery in Diamond hunt. Oil can be obtained by crafting an oil well or oil factory. Oil is used to power machinery and smelt ores.
Oil can be further increased by either achivement (mining easy achivement increases max oil by 500 or Silos.
Your own max oil amount is 100.
Oil silo
Oil silo - Holds max 350 oil. Costs 3 silver bars to make at crafting level 10.
Golden oil Silo - Holds 1000 oil. Costs the previous silo + 20 gold bars to craft at level 20