Halacha requires providing enough oil in the cups of the Menorah on Hanukah to sustain the candles for at least a half-hour. If one does not have enough oil to sustain all the candles for a half-hour, he must ensure to supply one candle with enough oil to burn for a half-hour. The basic obligation of Hanukah candles is to light a single candle each night; all the additional candles are lit as an enhancement of the Misva. Therefore, if one cannot light all the candles and have them burn for a half-hour, he should supply one candle with enough oil for half-hour, and he may then use the remaining oil for the additional candles. This way, he ensures to fulfill the strict obligation. Thus, for example, if on the fourth night a person does not have enough oil to sustain four candles for a half-hour, he should supply one candle with a half-hour’s worth of oil, and then use the rest of the oil for the other candles. If all the candles burn for less than a half-hour, one does not fulfill the Misva at all.
If a person has just enough oil for all the candles required that night, but his neighbor does not have any oil with which to perform the Misva, the one with the oil should share with his neighbor. Even though this will result in his being unable to perform the Misva at the highest standard, as he will now not have enough oil for all the candles, he should nevertheless share his oil to enable his neighbor to perform the Misva. It is preferable to compromise on the enhancement of a Misva, and perform it on the basic level of obligation, for the sake of enabling one’s fellow to perform the Misva, rather than fulfill the highest standard while one’s fellow cannot perform the Misva at all. Since one fulfills the basic obligation of Hanukah candles by lighting a single candle, it is proper to satisfy this basic requirement and forego on the enhancement in order to allow one’s fellow Jew to fulfill his obligation.
Thus, for example, if on the fourth night of Hanukah one’s neighbor comes and says he has no oil at all, he should give his neighbor oil for lighting, even though he will then be unable to light four candles. It should be noted that in this case one lights only a single candle, even though he is able to light three candles. Lighting three candles will mislead people into thinking it is the third night of Hanukah, and thus nothing is accomplished by lighting the second and third candles. Once a person is unable to light the proper number of candles, he should light just a single candle to fulfill his basic obligation.
(Based on Aruch Ha’shulhan, 671:10)
Summary: If a person does not have enough oil to sustain all that night’s candles for a half-hour, he must ensure to supply one candle with enough oil to last a half-hour, so he at least fulfills the basic obligation. If one has just enough oil for that night, but his friend does not have any oil, he should share some of his oil and light just a single candle, so that his friend can also fulfill the Misva.