Roman Catechism 20.8

Chapter 20 — Part II: The Sacraments — Extreme Unction

Section 8. The Matter Of Extreme Unction — Its element, then, or matter, as defined by Councils, particularly by the Council of Trent, consists of oil consecrated by the Bishop. Not any kind of oil extracted from fatty or greasy substances, but olive oil alone (can be the matter of this Sacrament).