Zevachim Chapter 10, Mishnah 8 - the last Mishnah in the chapter. Seemingly, this topic is found outside the flow of the chapter, and one must ask what its place is here. Apparently, since the previous Mishnah dealt with the eating of the sacrifices and the rules that apply to it, this Mishnah comes to deal - indirectly - with the eating of meal-offerings as well. The essence of the topic, although it does not appear so at first, is the question: may a person volunteer oil as a meal-offering on its own - without fine flour, but rather just a log of oil (about half a liter, meaning a jar of oil and not a bucket).
Although this is the central point of dispute, the structure of the Mishnah is different: Rabbi Shimon, who holds that there is no possibility of volunteering a log of oil on its own, comes to clarify what that oil could be which appears to be distributed to the kohanim to eat or going up on the Altar.
"Im ra'isa shemen shehu mischalek ba'azarah" - If you saw oil being distributed in the Courtyard:
The Mishnah says: "Amar Rabbi Shimon: Im ra'isa shemen shehu mischalek ba'azarah" - Rabbi Shimon said: If you saw oil being distributed in the Courtyard - oil that is distributed to the kohanim in the Courtyard, like a sort of remnants from the sacrifices. Since this oil cannot be an independent oil meal-offering, Rabbi Shimon establishes: "Ein at tzarich lishal mah hu" - you do not need to ask what it is - there is no need to ask what its source is, since the possibilities are limited to two:
"Ela mosar rekikin minchas Yisrael" - rather, it is the leftover of wafers of an Israelite's meal-offering - the remnants, the excess, from the oil that was used in the preparation of a wafer meal-offering brought by an Israelite who is not a kohen.
"V'log shemen shel metzora" - and the log of oil of a metzora - the remnants from the oil that the metzora brings.
What are these two cases? A meal-offering baked in an oven, which was brought by an Israelite who is not a kohen, can be prepared in two ways:
Chalos: This can be imagined as a Sephardic matzah - it is not chametz, but the texture is close to pita bread.
Rekikin: Like wafers or crackers, thin and cracker-like things, like an Ashkenazi matzah.
The meal-offering recipe: different meal-offerings are prepared in different ways, but their ingredients are almost always identical - a tenth of an ephah of fine flour, a volume of 43.2 eggs, which is a large amount, and a log of oil (about half a liter, about two cups).
With chalos, the two things are mixed together, made into dough and baked. But with rekikin, the dough is first baked in the oven, and only afterwards is the oil applied to it. There is a discussion of how the oil is smeared: the Bartenura understands that it is like a sort of Greek letter kaf, in its shape; others learned that the intention is the Greek letter chi, in the shape of an X. Either way, a sort of X of oil is placed on top of the rekikin.
In the opinion of Rabbi Shimon, after this smearing, oil remains. The kohen places the rekikin on the fire, and the rest is eaten; whereas the remaining oil is an entity in itself which is eaten separately - one can dip the bread in it. This is the oil that is given to the kohanim to eat.
The Sages disagree with this assumption: when preparing rekikin, one must add oil and more oil, until it is all absorbed in the rekikin, and therefore nothing will remain of it. According to their view, there is no question here to begin with, and such is the halachah.
The second possibility that Rabbi Shimon lists is "log shemen shel metzora" - the log of oil of a metzora. The metzora is punished with tzaraas for his anti-social behavior, and when he mends his ways and comes to be purified, he brings several sacrifices: a guilt-offering, a sin-offering, and a burnt-offering (we saw the guilt-offering as the central sacrifice not long ago, in this very chapter). At the end of the process, the blood of the guilt-offering is smeared on the lobe of his right ear - not in the ear canal but on its central part - as well as on the thumb of his right hand and the big toe of his right foot.
After this comes the oil: the metzora brings a log of oil, and the kohen sprinkles from it seven times toward the Holy of Holies. Afterward, a second kohen forms a sort of cup with his left hand, and the oil is poured into it; the first kohen dips his right index finger into the oil in his fellow's palm and places it on the same spots where he placed the blood - the middle of the right earlobe, the right thumb, and the right big toe. Whatever remains in the palm is smeared on the metzora's head, and through this he is purified. However, the kohen's palm does not hold an entire log, and therefore the remaining oil is given to the kohanim. This, says Rabbi Shimon, is the other possibility - since, according to his view, there is no generic oil offering that is allocated to a kohen to eat.
"Im raisa shemen shehu nasun al gabei ha'esh":
Rabbi Shimon continues: "Im raisa shemen shehu nasun al gabei ha'esh" - if you see oil that is placed upon the fire - meaning, one who sees kohanim placing oil directly onto the fires of the altar. Again, according to his view, there is no possibility for a person to volunteer oil by itself, and therefore, "ein atah tzarich lishol mah hu" - you do not need to ask what it is - since there are only two possibilities before him:
"Ela mosar rekikei minchas kohanim" - rather, it is the remainder of the wafers of a kohen's meal offering - the oil that remained from making the wafers that a kohen brought.
"Uminchas kohen hamashiach" - and the meal offering of the anointed kohen - the oil that remained from preparing the meal offering of the kohen gadol.
Regarding a kohen's meal offering, it says, "kalil tihyeh lo se'achel" - it shall be entirely burned, it shall not be eaten - every meal offering of a kohen is burned in the fire, and he is not permitted to eat any of it. Therefore, even though oil remains in the wafer recipe according to Rabbi Shimon, this oil is burned when it is a kohen who brought the wafers; and for this reason, one who sees oil going up on the fire - this is what is left over from the wafer process. However, as mentioned, the halachah ultimately follows the Sages, who say that they smear all the oil until it is entirely absorbed into the wafers.
The second possibility is oil left over from preparing the meal offering of the anointed kohen, which is the kohen gadol. Every day the kohen gadol brings the chavitin offering; and this applies to a kohen gadol regardless of how he was appointed - not only if he was anointed with oil, as we saw in Horayos, but even if he is only distinguished by additional garments, wearing the eight garments. Technically, he is not obligated to bring it himself, but rather to pay for it.
It is worth noting that the chavitin are first baked and then fried, and with a lot of oil. In the regular recipe - an issaron of fine flour, a volume of 43.2 eggs, a large amount, mixed with a log of oil - a very thick mixture is formed, with the texture of creamy peanut butter, where a fork stuck into it would stand up straight. But for the kohen gadol's chavitin the recipe is different: they use three lugin of oil, and they bake it first before frying, and therefore the dough does not absorb all the oil and some remains. After the chavitin are made, and half is offered in the morning and half in the afternoon every day, oil remains, and that oil is brought up - and must be brought up - onto the altar. This, says Rabbi Shimon, is that oil that you saw. For he explicitly rejects the possibility of a person bringing up oil as a voluntary offering: "she'ein misnadvin shemen" - for people do not volunteer oil - there is no such thing as volunteering an offering of oil by itself.
The View of Rabbi Tarfon:
Rabbi Tarfon disagrees: "misnadvin shemen" - people do volunteer oil - there is such a thing. Just as a person volunteers a meal offering of fine flour, so too he can volunteer an oil offering, which is entirely oil. In the Gemara, there is a dispute regarding its halachah:
According to one opinion, and this is how the Bartenura learns: all the oil is burned on the fire and nothing remains of it.
According to the view of Shmuel, which is ruled as halachah by the Rambam and the poskim: not all the oil is poured onto the fire, but rather the kohen takes a kometz measure - a handful of oil - and salts it and places it on the altar; and the rest, since they bring at least a log of oil and the vessel is full, is given to the kohanim to eat.
Therefore, the halachah is: one who sees oil being distributed in the courtyard to the kohanim - this could be oil left over from a voluntary offering, since people do volunteer oil, or leftovers from the metzora's purification process. And one who sees oil going up on the fire - this could be a voluntary oil offering in the measure of a kometz, and even leftovers from the kohen gadol's chavitin, since oil remains there. But it will not be oil left over from making a kohen's wafers, because there the halachah is that they absorb all the oil into the wafers themselves.
A Voluntary Wine Offering:
The Bartenura adds an interesting point here: just as one can bring a voluntary offering of pure oil, so too one can voluntarily bring pure wine - at least three lugin - as a wine offering, and pour it on the Altar. Seemingly, one must ask: how can wine be poured on the Altar, since it extinguishes the fire, and there is a prohibition against extinguishing? Rather, the Bartenura says, the halachah follows Rabbi Shimon - not the Rabbi Shimon in our Mishnah, but in a separate dispute - who holds that a davar she'eino miskaven (an unintentional act) is permitted: when a person performs an action whose intended result is permitted, even though it is accompanied by a forbidden result. Here, his intention is not to extinguish the flames, even if they are slightly extinguished, but solely to bring the wine as a voluntary offering, and therefore it is permitted - provided it is not a pesik reisha (an inevitable outcome).
The bottom line is that the topic of davar she'eino miskaven is complicated and subject to dispute, and this is not the place to elaborate on it; but the practical halachah here is that it is permitted to bring the wine to the Altar as an independent gift.