Handbook of Liturgical Practice of the Russian Church Abroad
Despite the fact that many people, desiring piously and rightly to present what is best to the Church, assume virgin or extra virgin olive oil is the best type, since it is generally considered to be the best for cooking, it is not actually the finest oil for use in church lamps. The best oil to be used in lamps is non-virgin olive oil, which burns more brightly, with less flickering, and does not so swiftly clog up wicks and floats.
Explanation: Virgin olive oil is considered best for cooking since it is the oil that flows off of crushed olives without being pressed, and therefore contains an abundance of minerals and other elements that enhance flavour, but which clog up lamps and create dull or flickering flames. Non-virgin olive oil is produced after this initial drainage, by pressing the olives — by this time the minerals and other elements have already flowed away and what is left is an oil freer of these, which is generally considered less flavourful, but in terms of lamps burns much more cleanly and brightly. This is a rare occasion on which what is more expensive is not actually what is best for Church use.