to provide an additive for crude oil recovery , used in secondary / tertiary recovery of crude oil , which has high heat resistance and high mechanical shear resistance , of which the viscous property does not degrade even under high - temperature and high - shear conditions ; has high pseudoplastic flowability and therefore has a light transfer workload ; and is a naturally - derived one and is therefore highly biodegradable and has least adverse impact on environment . an additive for crude oil recovery , which contains cellulose fibers having a number - average fiber diameter of from 2 to 500 nm and a fiber aspect ratio of 50 or more and having a cellulose i - type crystal structure , and , in the cellulose fibers , a hydroxyl group on a surface of the cellulose fibers is preferably chemically modified .