Lantern Oil is the fuel for your lantern. It is found in small bottles scattered throughout the castle. Each bottle provides a limited refill. Given the scarcity of oil and the constant need for light, every drop is precious. Always be on the lookout for oil bottles, especially before entering long, dark sections. Prioritize collecting oil over other non-essential items in your limited inventory. Running out of oil in a dark, monster-infested area is a death sentence.
