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.

In Genesis 47:31, the bed signifies things of the interior natural, to which the Lord turned Himself. (Arcana Coelestia 5721)

In Exodus 8:3, this signifies inmost things. (Arcana Coelestia 7354)

In Revelation 2:22, this signifies doctrine which everyone acquires either from the Word, or from own intelligence; for the mind rests there, and sleeps, as it were. (Apocalypse Revealed 137)

(Odkazy: Amos 4, 6, 6:4; Apocalypse Explained 1146)