Gemara: Rabbi Tarfon says: We light [for Shabbat] only with olive oil.

Ein Ayah: The light of the Shabbat candles serves as a hint about the light of the intellect/spirituality that flows to the spirit that becomes sanctified in the day’s rest and sanctity.
There are many types of oil, which are the causes for the ability to create light. Similarly there are many branches of wisdom that, when the spirit is involved in them, elevate the person in ethics and good characteristics and make him able to bring goodness to the world.
However, the highest level of light of wisdom is that of Torah, and it is this light that is uniquely appropriate for Shabbat. It is pure and is not polluted with elements of mistake or the darkness of the evil inclination, as other types of human wisdoms are. That is why the Torah is represented by olive oil, which is the most important oil, to show that Shabbat should be a day of total sanctity and study of Torah, as the Rambam (Shabbat 30:10) says. The teaching that we are to refrain from other oils which give off less clear light shows that we should concentrate on the hidden parts of the Torah, where the purest light is found.