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.
Exodus 27:20-21
20
"You shall command the children of Israel, that they bring to you pure olive oil beaten for the light, to cause a lamp to burn continually.
21
In the Tent of Meeting, outside the veil which is before the testimony, Aaron and his sons shall keep it in order from evening to morning before Yahweh: it shall be a statute forever throughout their generations on the behalf of the children of Israel.
