Oil-soluble ionic liquids (ILs) have recently been demonstrated as effective lubricant additives in friction reduction and wear protection for sliding contacts, however their effectiveness and mechanism in mitigating rolling contact fatigue (RCF) are little known. Because of the distinct surface damage modes, different types of surface protective additives are used in lubricating the sliding and rolling contacts. As a result, the lubricating characteristics and mechanisms of ILs learned in sliding contacts from the earlier work may not be directly translatable to rolling contacts. This study investigated the feasibility of three groups of ILs, phosphonium-phosphate, ammonium-phosphate, and phosphonium-carboxylate, as candidate additives for rolling-sliding lubrication and tesults suggested that an IL could be either beneficial or detrimental on RCF depending on its chemistry. Specifically, the phosphonium-phosphate IL was the best performer and a 2% addition made a low-viscosity base oil significantly outperform a more viscous commercial gear oil in all aspects including friction, sliding wear, RCF cracks, as well as vibration. Worn surface characterization revealed a thicker, smoother, and more uniform tribofilm formed by this IL compared with that grown by the conventional additives, which is believed to provide a better cushion for contact and rubbing, leading to a superior protection for both sliding abrasion and RCF.