Oil – typically olive oil – was an extremely important product in Biblical times, for food preparation, medicinal ointment and for burning in lamps. As such, it is fitting that it has such an important meaning, what Swedenborg calls "the good of 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.