ד "יז" הנותר ביום השמיני "יח" מן השמן הצריך לשיעור הדלקה עושה לו מדורה "יט" ושרפו בפני עצמו שהרי הוקצה למצותו ואם נתערב בשמן אחר ואין ששים לבטלו יש מי שאמר "כ" שאין להוסיף עליו כדי לבטלו
4: [17] Any oil left over on the eighteighth day that was needed for the shiur of lighting was be made into a "bonfire" [19] and burnt by itself because it was "separated" for the purpose of the Mitzvah and if it was mixed with other oil, and there is not sixty parts to nullify it there are those that say [20] that one cannot add to it in order to nullify.
Good question. What are you to do with the leftover oil or candle stubs? The issue at hand is that fuel set aside for the mitzvah of Chanukah lights is now considered sacred and should not be used for anything else.
But what did you actually "set aside"? Technically speaking, you fulfill the mitzvah of lighting the menorah once your lights have been burning for a half-hour after nightfall. Thus, the Code of Jewish Law1 tells us that whatever is left over from the oil that was to burn in that half hour—though the flame was somehow extinguished—is consecrated for the mitzvah and should be burnt after Chanukah to ensure that it is not used for mundane purposes. If, however, you filled your cups with so much oil or used such big candles that they burnt for longer than the required time and you still had leftover fuel, you are free to do whatever you want with those leftovers as they were never "set aside" for the mitzvah lighting.
Other authorities,2 however, maintain that any leftover fuel that was placed in your menorah should be treated as "set aside," and burnt. For this reason, it is customary to burn all the leftover wicks, oil or wax from your menorah after the holiday is finished.3
Obviously, this would not apply to the oil still in the bottle and the candles still in the box, which may be used in whichever manner you see fit. FOOTNOTES
FOOTNOTES
Orech Chaim 677:4.
Cited in Mishnah Berurah ad. loc.
Incidentally, there are those who have the custom to burn these leftovers with the chametz on the morning before Passover.