Menachos 89a - Amount of oil in Menorah

The gemara says that the Rabbonon determined that the amount of oil necessary for each candle in the menorah was 1/2 lug of oil per night. This amount would be the right amount for even the longer nights of the winter. However, there is a machlokes in the gemara how they figured out that this was the right amount. One opinion says that began with a larger amount and when they found too much oil left over in the morning, they slowly reduced the amount that they initially put in the candle - the advantage to this approach is אין עניות במקום עשירות, we aren't concerned with "waste" in the beis hamikdash. The other opinion says that they started with a smaller amount of oil (rashi says that they began with a rivi'is - not clear where he gets that from), and when they realized that it didn't last the entire night, they slowly started to add oil - the advantage to this approach is התורה חסה על ממונן של ישראל , not to waste oil. The opinion who says that they started with a small amount of oil and when they realized it wasn't enough, they would add on the future nights, is very difficult to understand. How could they have been cheap on the oil in order to save money since it could lead to not fulfilling the mitzvah of having the candles burn all night long. This gemara seems to imply that the mitzvah of lighting the candles in the beis hamikdash was הדלקה עושה מצוה, similar to what we say by ner chanuka. However, the gemara and rambam clearly hold that the mitzvah was to hvae the candles burning all night long and the mitzvah is not fulfilled at the moment of lighting. Furthermore, even by chanuka the halacha demands that at the time of lighting there is sufficient oil to burn for the duration of the time. This should certainly be necessary by the candles in the beis hamikdash. How can there be an opinion that they began with a smaller amount of oil, and not fulfill the mitzvah of lighting the menorah?

The Nesivos HaMishpat (Chavos Da'as) has a sefer with hagahos on shulchan aruch and its commentaries called M'ginei Artetz. In Hilchos Chanuka he writes:

בספר מגיני ארץ מבעל נתהמש"פ על הל' חנוכה החליט דבמנורה שבמקדש לא אמרינן שהדלקה עושה מצוה וז"ל לכן נראה דמצוה דהדלקה דחנוכה אינו דומה למצות הדלקה דמנורה, דבנר חנוכה אין המצוה רק תחלת ההדלקה שידליק שיעור כזה, תדע דהא כבתה אין זקוק לה ומש"ה צריך לכתחלה ליתן כשיעור, משא"כ במנורה המצוה והחיוב הוא שיהיה דולק כל שיעור הזה דהא כבתה זקוק לה ומש"ה סגי אפילו כשנותן לכתחלה פחות מכשיעור רק שיראה להוסיף בכל פעם עד שידליק כל השיעור וכמ"ד ממטה למעלה (היינו מ"ד במנחות פט ע"א שמתחלה נתנו רביעית ואח"כ הוסיפו עד חצי לוג כשראו שאין סיפוק). ולפי"ז אתי שפיר בס' תרע"ג ס"ב דאמר הדלקה עושה מצוה לפיכך אם כבתה וכו' שעמד עליו הט"ז בס"ק ח' (שהקשה מה הקשר בין הדלקה עושה מצוה לכבתה אין זקוק לה, הא אף אם הנחה עושה מצוה נמי נוכל לומר שאם כבתה אין זקוק לה). ולפמש"כ אתי שפיר כיון דאין מצותה שיהיה דלוקה כל שיעור הזה כמו במנורה, רק תחלת ההדלקה לפיכך אם כבתה וכו' ואתי שפיר עכ"ל. הרי סובר דהא דנקט המחבר לשון "הדלקה עושה מצוה" לאו לאפוקי הנחה אלא לאפוקי המנורה שבבית המקדש שהמצוה לא נגמר במעשה הדלקה אלא המצוה היא שתהא דולקת ועומדת.

The Nesivos explains that the opinion who says that they started with a smaller amount of oil doesn't mean to say that they put a rivi'is in the first night and when they saw it went out early they added a little the next night. Rather, they started with a rivi'is and when they realized at 4am that it wasn't going to last until the morning, they added a little more. Then at 5am when they saw it wouldn't last they added a little more, until they realized that it would take a half lug to stay burning until the morning. This is actually very medu'yak in rashi who writes:שבתחלה נתנו רביעית ועמדו שם וראו שאין סיפוק ושוב הוסיפו עד חצי לוג. Rashi clearly holds that they stood there all night adding oil until they figured how much would be needed. The nesivos explains that this was av viable option since in the beis hamikdash we don't say הדלקה עושה מצוה, so that adding oil after it is already burning is acceptable. However, Rashi in the k'sav yad clearly writes: בתחלה רביעית הלוג, ולא דלק אלא חצי הלילה, ליל שני הוסיפו וכן כמה לילות עד ששיערו חצי לוג ללילה. Rashi in the k'sav yad holds that they would add more the next night. According to this approach it is very difficult how they could be meva'tel the mitzvah the first few nights just to save a few pennies! Rabbeinu Gershom seems to say that they originally thought that less than a half lug would be sufficient. They never made this decision in order to save money, rather because they thought that it could indeed burn all night. Eventually they realized that a half lug was necessary to burn all night, but we learn from the fact that they initially put in what they thought is the bare minimum that the Torah cares about not wasting money and it is therefore "not permitted" to add more than a half lug.