Ionic liquids (ILs), both as pure lubricants and as lubricant additives, have been demonstrated extensively to exhibit excellent tribological performance in terms of friction and wear reduction in the boundary lubrication (BL) regime. Because engineering contacts experience boundary and mixed as well as full film lubrication depending on operating conditions, it is crucial to examine whether lubrication regimes other the BL regime can also benefit from the use of ILs. The objective of this work is to investigate the tribological performance of IL additives in the mixed lubrication (ML) and the elastohydrodynamic lubrication (EHL) regimes. Polyethylene glycol (PEG) was used as the base fluid. ILs were synthesized in situ by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PEG. Friction and film thickness measurements were employed to investigate the effectiveness of IL additives at room temperature, 60°C, and 80°C at various loads and slide–roll ratios (SRRs). The effect of IL additives on the rheological behavior of PEG was also investigated. The EHL film thickness increases with increasing IL concentration. EHL friction is, however, only mildly affected by IL additives. In the ML regime, IL additives can reduce friction and metal wear compared to pure PEG in mild conditions. It is conjectured that IL forms sacrificial layers and protects the rubbing surfaces.