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.

2507. 'Of Sarah his wife' means spiritual truth joined to the celestial. This is clear from the meaning of 'Sarah his wife' as intellectual truth joined to Divine Good, or what amounts to the same, spiritual truth joined to the celestial, dealt with in 1468, 1901, 2063, 2065, 2172, 2173, 2198. What the spiritual is, and what the celestial, has been stated quite often already, see 1155, 1577, 1824, 2048, 2088. That is called the celestial which consists in good, that is, which consists in love to the Lord and in charity towards the neighbour; and that is called the spiritual which consists in truth, that is, which consists in faith deriving from love and charity.