Oil
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.
Numbers 16:46-48
46
Moses said to Aaron, "Take your censer, and put fire from off the altar in it, and lay incense on it, and carry it quickly to the congregation, and make atonement for them; for wrath has gone out from Yahweh! The plague has begun."
47
Aaron did as Moses said, and ran into the midst of the assembly; and behold, the plague has begun among the people: and he put on the incense, and made atonement for the people.
48
He stood between the dead and the living; and the plague was stayed.