Or HaChaim on Exodus 29:7/אור החיים על שמות כ״ט:ז׳

1 ויצקת על ראשו ומשחת אוהו, "and pour it upon his head and anoint him." The verse means that the oil had to be poured on his head. The same applied to the anointing of Aaron's sons. Aaron and his sons each required a separate pouring of oil upon their heads seeing that the verse referred to both of them in the same way as we know from 28,41. This is further reinforced by the wording in 40,15 where the Torah said: "you will anoint them just as you have anointed their father. Torat Kohanim on Parshat Tzav explains the meaning of that verse in the same spirit.