Sifrei Mitzvot sobre Levítico 5:6

Not to put olive oil in a sin-offering made of flour, as it is said, “He shall put no oil upon it” (Lev. 5:11).

To not place olive oil in the meal-offering of a sinner: That a priest not place oil in the meal-offering of a poor sinner, as it is stated (Leviticus 5:11), “he shall not place oil on it” — and even though they would put oil on other meal-offerings. And I said, “the meal-offering of a poor sinner,” because of [the following]: since a rich sinner never brings a meal-offering of flour, but rather a sacrifice of a beast, as is explicit in the Torah.

It is from the roots of the commandment [that it is] because oil is a hint for stature and greatness — since if you mix it with any liquid, it floats above them all; and it is a very significant thing. And the significance of good oil is well-known — and therefore they would anoint those inaugurated to the stature of monarchy or priesthood with it. Hence it is not fit to place [some] of it in the meal-offering of the sinner who needs to show concern and lowliness about himself, that the matter of a sin came to his hand. And we can further say [it is] from compassion for the poor person, that he need not burden himself more than necessary to bring oil — as God, blessed be He, does not burden a creature. And because of this, it also obligated him only a little flour — as it is impossible for any person, even the most destitute of the destitute, [not to have] a little flour. And this will suffice for us also about the frankincense.