Difference between Chanukah & Shabbos candles
The Mishna Verurah (sk 18) writes that מעיקר הדין the only oil that would be הוקצה למצותו is the exact
amount of oil needed for the שיעור. If this oil was leftover because the candle extinguished early then
the oil is Muktzah.
Extra oil poured into the Menorah which is not needed for the שיעור is not הוקצה למצותו מעיקר הדין.
The M.V. continues "There are Poskim who hold even if the extra oil becomes Muktza unless he made a תנאי.
שסוברים דאם נתן בסתם הקצה למצותה הכל:"
Perhaps the reason Poskim hold this way is that since there are 8 days Chanukah, therefore, people intend to light
the following night with the oil left over from the previous night.
By continuing to reuse the leftover oil every night it might be considered as if he was מקצה it בפירוש
and all oils in the Menorah becomes Muktzah.
Whereas Neiros Shabbos oil is only filled for one day use therefore on the leftover we apply the עיקר הדין and only the
minimum שיעור is הוקצה למצותו (doesn't exist anymore) and the rest that is leftover is not Muktzah. אמבצי