Oil -- typically olive oil -- symbolizes the good that comes from celestial love. Celestial love is love of the Lord, the highest and purest love we can have. The good of celestial love is the desire to be good springing from celestial love: Wanting to do the Lord's will because you love Him. That's not a state many reach, even in heaven, but it's a beautiful goal. This representation makes sense based on the idea that the sun -- the source of all natural light and heat, and thus all natural life -- represents the Lord, the source of all spiritual light, heat and life. Burning oil was the most pure fire available in Biblical times, thus the closest representation of the sun people could create.

7083. Heat and light in the natural world emanate from the sun in the world, whereas spiritual heat and spiritual light, or love and faith, emanate from the sun in heaven. The Lord is the sun in heaven, and the heat which comes from Him as the Sun is love, while the light which comes from Him as the Sun is faith. The fact that the Lord is light is clear in John,

Jesus said, I am the light of the world. He who follows Me will not walk in darkness but will have the light of life. John 8:12.

And the fact that the Lord is a sun is clear in Matthew,

When Jesus was transfigured His face shone like the sun, and His clothes became [white] as the light. Matthew 17:1.