Sefer HaChinuch ספר החינוך

That we not put oil on the sotah sacrifice: That we not put oil on the sotah sacrifice, as it is stated (Numbers 5:15), “And he shall bring her sacrifice, etc. and he shall not pour oil upon it.”

From the roots of the commandment is that which they, may their memory be blessed said (Bamidbar Rabbah 9:13) [that it is] so that her sacrifice not be beautiful. As oil is called light and she did it in the dark; and she did the act of an animal which fornicates with all, therefore her offering is inferior, it being barley (Sotah 15a). And Rabbi Moshe ben Nachman, may his memory be blessed, also wrote (Ramban on Numbers 5:15) regarding the matter of the sacrifice of the sotah, on the level of its simple meaning, that the husband brings this sacrifice from his [possessions] for the jealousy that he has towards her, so that God, may He be blessed, will take vengeance from her. And the reason for the barley (seorim) is that there be “an angry storm (saarah) of the Lord going out, and the storm land on the head of the evildoer, that it land on the head” of the unfaithful woman; and like the matter of griddle bread of barley that is stated with Gidon (Judges 7:13), which they interpreted as a great, tumultuous storm. Likewise, [it being] in the clay vessel is a sign that she should be broken like the pot of a craftsman; likewise, the dirt is a sign that she will die and return to the dirt. But regarding the matter of the oil, he did not say anything. And I will answer my portion about it: [It is] because the oil is the highest of all the liquids, as it floats over all of them. And now that the sotah has corrupted her deeds and he has removed her from being the mistress and put her down to be lowly and contemptuous, it is not fitting to bring on her sacrifice the glorious oil which is prepared for light and greatness — to anoint the kings and the high priests with it. And the principle of the matters that we have been commanded [is that they are] to refine the thought of those that do the process, and to place into their hearts that one who corrupts his actions [will have] all of his ways come to disgrace and shame, and [be] “full of humiliation” and mockery; and that God will benefit the good, and “joy and gladness and preciousness will reach them.” This commandment with its laws is elucidated in the Scripture and there is no need to write at length about it. (See Mishneh Torah, Laws of Woman Suspected of Infidelity 3.)

And it is practiced at the time of the Temple. And as we wrote above in this Order in the commandment of the sotah (Sefer HaChinukh 365), we only have a sotah drink with the court of seventy-one in the Temple. And a priest that transgresses this and pours oil on the offering of the sotah has violated a negative commandment and is lashed.