The Shulchan Aruch (677:4) writes:הנותר ביום השמיני מן השמן הצריך לשיעור הדלקה עושה לו מדורה ושורפו בפני שהרי הוקצה למצוותו עצמו - “For what remains on the eighth day of the oil necessary for the measure for kindling one should make a bonfire for it and burn it by itself, for it has been set aside for its mitzvah”. (This is the opinion of most Rishonim, however, there is a Shiltay HaGibborim (Shabbos 9a, Midafay HaRif) which says that on Chanukah any leftover oil can’t be used, however, after the eight nights of Chanukah have finished, and the time of the mitzvah has passed, then one may benefit from any leftover oil.) The Shulchan Aruch continues:
ואם נתערב בשמן אחר ואין ששים לבטלו יש מי שאומר שאין להוסיף עליו כדי לבטלו – “And if it becomes mixed with other oil and there is not sixty parts to nullify it, there is an opinion that says that one should not add to it in order to nullify it.”
The Chiyuv [Obligation] Is Only on Oil That is Leftover from the First 30 Minutes
From the lashon [language] of the Shulchan Aruch, מן השמן הצריך לשיעור הדלקה – “of the oil necessary for the measure for kindling”, it’s clear, that only oil which is leftover from the first half-an-hour (which is the minimum amount of time the menorah has to be kindled) must be burnt.
The Mishnah Berurah (s.k. 18) explains that if one put lots of oil in the menorah and there is still some leftover one is allowed to use it lechatchilah [ideally]. The Shulchan Aruch (672:2) says that if one puts in extra oil, it’s ok to extinguish the candles after half-an-hour, similarly, one is allowed to benefit from the candles after this time. Since one may use the oil, it doesn’t become forbidden to benefit from, therefore, one may use it.
However, the Mishnah Berurah adds, that there are a number of poskim (Bach end of siman 677, Pri Chodosh s.k. 4, Mekor Chaim 672:2, Sheilas Ya’avatz 1:4, and Yashras Ya’akov 672:3) who hold that if one puts into the menorah oil, unless one stipulates clearly beforehand that only oil needed for the first half-an-hour is set aside for the mitzvah, automatically the oil becomes set aside for the mitzvah and forbidden to benefit from. Due to these poskim, the Mishnah Berurah (672:7) writes in the name of the Magen Avraham, that lechatchilah one should stipulate that he only wants to set aside for the mitzvah oil that is needed for the first half-an-hour.
The Biur Halachah (677:4 d.h. min hashemen) writes, that sometimes there is a chiyuv to burn any oil that remains from the eighth day of Chanukah, even when making a stipulation. If on one of the days of Chanukah one of the Chanukah candles went out within the first half-an-hour, the oil is הוקצה למצוותו, set aside for its mitzvah, and generally speaking, when one adds more oil to it on subsequent nights, it’s not enough to nullify it by sixty. Consequently, in such a case, even if one makes a stipulation it won’t help, and the remaining oil must be burnt.
Keeping It for Next Year
The Mishnah Berurah (s.k. 19) says, it’s forbidden to keep the remaining oil for next year, as we are worried for takolah, that a mishap may happen, and one will accidently end up using it for something other than Chanukah candles. Bedieved [post-facto] however, if one did keep the oil for another year, one may use it for his Chanukah candles.
