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.

3728. 'And poured oil on the top of it' means holy good [in which it originated]. This is clear from the meaning of 'oil' as the celestial element of love, which is good, dealt with in 886, 3009, and from the meaning of 'the top' as that which is higher, or what amounts to the same, that which is interior - good being that which is higher or interior, and truth that which is lower or exterior, as has been shown in many places. From this one may see what was meant by the ancient practice when people poured oil on the top of a pillar, namely that truth should not be devoid of good but should be grounded in good, thus that good should rule, like the head on top of the body. For truth devoid of good is not truth but is a meaningless sound and the kind of thing that is reduced to nothing. In the next life it is so reduced even with those whose knowledge of truth or matters of doctrine concerning faith, and with those whose knowledge of matters of doctrine concerning love, has been superior to anybody else's, if they have not led a good life and so have not out of a desire for good held on to truth.

[2] Consequently the Church is not the Church by virtue of truth separated from good, nor therefore by virtue of faith separated from charity, but by virtue of truth that is grounded in good, or faith that is grounded in charity. The same is also meant by what the Lord said to Jacob,

I am the God of Bethel, where you anointed a pillar, where you made a vow to Me. Genesis 31:13

Also,

Jacob again set up a pillar, a stone pillar, and poured out a drink-offering over it, and poured oil over it. Genesis 35:14.

'Pouring out a drink-offering over the pillar' means the Divine good of faith, and 'pouring oil over it' the Divine good of love. Anyone may see that unless it meant something celestial and spiritual, pouring oil over a stone would be a ridiculous and idolatrous action.