Charles Ellicott Commentary Exodus 40:15

"and thou shalt anoint them, as thou didst anoint their father, that they may minister unto me in the priest`s office: and their anointing shall be to them for an everlasting priesthood throughout their generations." — Exodus 40:15 (ASV)

You shall anoint them, as you did anoint their father.— By the narrative of Leviticus 8, it would seem that Aaron’s sons were not anointed in the same way as Aaron himself. He had the oil poured over his head (Leviticus 8:12; Psalms 133:2). They were merely sprinkled with a mixture of oil and blood (Leviticus 8:30). The difference implied a lower degree of official holiness.

Their anointing shall surely be an everlasting priesthood. —The Jewish commentators maintain that the one anointing of the sons of Aaron sufficed for all time, and was not repeated in the case of any subsequent priests, who became fully possessed of the sacerdotal office without it. Each high priest was, on the contrary, inducted into his office by anointing, from which the high priest came to be spoken of as “the anointed priest” (Leviticus 4:3; Leviticus 4:5; Leviticus 4:16; Leviticus 21:12, and elsewhere).