“Now you shall command the Children of Israel and have them bring you pure olive oil, pressed for lighting, to keep the lamp burning constantly.” 27:20
o Pressed - He would crush the olives in a mortar and not grind them in a mill so that it would not contain sediment. After extracting the first droplet, he would put them into a mill and grind them. This second oil is unfit for the menorah but may be used for meal offerings for it states: "pressed for lighting," but it need not be pressed for meal offerings. - Rashi
- Having completed the commandments regarding the actual construction of the Mishkan, the Torah now turns it’s attention to those who would perform the service within it. It begins by teaching that the oil for the Menorah must be absolutely pure and may not even contain sediment. This requirement of absolute purity is a fitting prelude to the designation of Aharon and his sons as Kohanim, since they too, must remain pure and separate from the rest of the nation. They, and only they, were permitted to partake in the service. – Ibn Ezra
The mention of the requirement of pure oil prior to discussing the designation of Aharon and his sons was also fitting in the sense that they were being taught that illumination must only come from the purest of sources. As kohanim, they were expected to act as leaders and teachers of the nation. Their effectiveness in this capacity would depend on the level of purity with which they conducted themselves. It was imperative that they always maintain the highest standards of personal morality as any failing in this regard would bring about disastrous consequences.