"It's fuel for a lighter."
In Resident Evil 3: Nemesis this item can be found in the basement at the Y-shaped alley. This is one of necessary items for creating the Lighter. After the combining with the Empty Lighter, the oil disappears from the inventory.
The fuel used in the games' lighters is correctly stated to be "fluid" in the first Resident Evil game, but it's wrongly called "oil" in every subsequent game it's mentioned.