It is an object of the present invention to provide an additive having strong thickening and gelling effects, the additive being capable of thickening and gelling silicone oils and ester oils even when added in a small amount. To achieve this object, the present invention provides, as the additive, a polyurethane polyurea obtained by reacting a polyurethane polyurea prepolymer obtained from a silicone derivative starting material having a specific structure, a specific diisocyanate compound starting material and a starting material made up of a compound represented by General Formula (C) below (each of which is set forth in the Description), with either or both of a nitrogen compound having a specific structure and a hydroxyl group-containing compound having a specific structure (each of which is set forth in the Description): where R 30 represents a C 1-10 divalent hydrocarbon group or oxygen atom, and y represents a number 0 or 1; however, when R 30 is a C 1-10 divalent hydrocarbon group, y represents the number 1, and when R 30 is an oxygen atom, y represents the number 0.