Exodus 27:20

And thou shalt instruct the sons of Israel to bring the pure olive oil, beaten, for illumination, that the lamps may burn continually.

You shall command the B’nei Yisrael. The word “command” indicates that this instruction applies for all generations. That is, the oil of the lamp must always be purchased with public funds. Thus the oil brought by the leaders of the tribes could only be used for anointing.

Scripture states here, and thou shalt command [and does not say, “command the children of Israel”], because He always said with reference to the Tabernacle and thou shalt make, which means by your command [i.e., “you Moses command them to do it”]. Therefore He said here “and you yourself command them that they bring to you the oil for the light.” [It says that they bring and not “that they make”] because they had no way of making the oil in the wilderness; [therefore they were told] to bring it if they had it in reserve. Actually it were the princes that brought it.

The meaning of the word eilecha (unto thee) — [that they bring ‘unto thee’ pure olive oil] is that they were to bring it to Moses and he would see if it was pure and beaten properly. A similar intent is present in the verse, Command the children of Israel, that they bring unto thee [pure olive oil beaten for the light].

ואתה תצוה, whereas in 25,2 Moses had been told by G’d to raise the donations for the building of the Tabernacle with the words ויקחו לי תרומה, and that had referred to a one time contribution, now Moses is instructed of the ongoing obligation to provide oil for the Menorah in the Tabernacle. The syntax has been changed here with the introduction of the word תצוה, “you will command.” Whenever that root occurs the subject matter it addresses is one of ongoing importance, not a commandment to be carried out only once, only because of special circumstances. This principle has been spelled out by Torat Kohanim in Leviticus at the beginning of Parshat Tzav, as well as in Sifrey on Parshat Nasso section 1.

זך, free from sediments. The olives are to be crushed in a mortar for if they were to be ground in a mill the result would not be satisfactory.

ואתה תצוה, If, prior to this admonition, the Torah introduced its instructions with the words ועשית, “you will do, etc.,” the meaning had been that Moses was not to do this personally, but that he was to delegate the tasks in question. The formulation ואתה, however, means: “and you personally.” He, personally, was to issue instructions to the Israelites concerning providing the oil for the Menorah. He, personally, was to assemble Aaron and his sons and to address all the חכמי לב, “wise-hearted people.” (28,1 and 28,3)

ויקחו אליך, as the need would arise in the future, after the Tabernacle had been completed. The people should not think that donating oil for the Menorah now was the only time that this commandment would be actual. It would have to be replaced from time to time as the supply kept would become depleted on an ongoing basis.