Rav Yosef Jacobovitz
Yitzchak brought Rivka home and saw a special blessing in Rivka’s mitzvah of hadlakas neiros, similar to his mother, Sarah, in that it was not extinguished from shabos to shabos.
What is the minimum requirement as a fuel?
Shulchan Aruch – all oils and waxes are permitted; yet, olive oil is preferable based on mesorah.
Mishna Berurah – olive oil is preferable only because it kindles well. If olive oil isn’t readily available, one may use wax candles.
Rav Moshe Feinstein- the clearest flame is the most preferred.
The Steipler Gaon – used wax candles since it lit better or equal to oil.
The Baal HaTurim – Parshas Tetzave- Tetzave is the gematria of nashim tzavei– that they too should light with oil.
Rav Shmuel Kamenetzky- based on above, one would need hataras nedarim to change their minhag from olive oil to candles. However, if one does not have a particular minhag, it is not necessary to use specifically olive oil.
When one cannot light [i.e. a hospital]
Electric bulbs: Rav Chaim Ozer Grodzensky- an incandescent bulb is a regular flame.
Rav Henkin agrees.
Rav Shlomo Zalman Auerbach distinguishes between a battery operated device [i.e. a flashlight] where the entire energy source is present and ignited prior to shabos. A wall based ac current has only a small portion of the energy at the point of the outlet.