Shulchan Aruch 677:4
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:  Any oil left over on the eighth day that was needed for the
shiur of lighting was be made into a "bonfire"  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  that one cannot add to it in
order to nullify.
Note I do not have the time to copy the original Mishna Brurah so I just put in my translation.
Mishanah Brura says
 On the previous nights any unused oil can be used for the mitzvah
of lighting on the next night.
 This occurs when he did not put in more than the shiur of
lighting (half an hour) and it was extinguished within that period and
therefore it was separated for the purpose of the mitzvah. but if he
put in a lot into the cup and there is extra it is mutar to use it
lechatchilah as is written in earlier in siman 672. Know that there
are poskim who rule that if he puts in oil with no specification
all the oil in the cup is considered is kodesh as long as he did not
specify in advance that the extra (above half an hour) is not to
be considered kodesh. See Mishna Brurah Siman 672 seif katan 7
 To leave it for the next year's chanukah lighting is asur because
we are afraid of an error (that would use it for something else) and
even if you put it in a "disgusting" vessel so that he will definitely
not eat it even so it is asur since we are afraid that he may come to
light it (for a secular purpose) and have han'ah from it.
 See Yoreh Deah siman 99 seif 6 that their are those that allow in
every isur of the rabbi's to add to a mixture (enough to make the isur
batel) and that is why it says יש אומרים
The Mishnah Brurah explains that if oil is put into the cup, then the "extra" oil is not kodesh and that the "extra" oil can be used for secular purposes (such as latkes) once the first half hour of oil has been burnt. However, there are poskim who say that if the oil is put into the cup without a tnai (specification) that only the first half hour of oil is kodesh then the entire contents of the cup is kodesh from the beginning.
Once the oil is extinguished, if there is a mixture of kodesh and chol in the cup (whichever of the two views is being considered), then it can be used only if the kodesh is batel beshishim. There are those who say that other oil can be added to bring it to the bateil beshishim but in Yorah Deah 99:6 there are those who say that extra oil cannot be added to make it batel.
The specific answer to your question would be that you would need to make the specification at the time that you put the oil in the cups in order to ensure that the "extra" oil is usable according to everybody and that it has burned beyond the time specified.
Note that this does not deal with the case of someone having made latkes and then using the oil for the candles (whether it was new oil or allowable oil from the previous night's menorah). That is a separate question.
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
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
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.