## 1 Introduction  
Lubricants enable the efficient and reliable operation of moving mechanical assemblies by reducing friction and wear. Ideally, a lubricant film fully separates the moving surfaces so that wear is minimized and friction is limited to shearing of the viscous film. Unfortunately, there are many cases where the fluid film cannot be maintained and the asperities of the surfaces come into direct contact (i.e., the system operates in the mixed or boundary lubrication regime). To avoid excessive wear in these regimes, wear and friction reducing additives are required. The past century has seen the development of an extensive set of lubricant additives that can significantly improve the tribological performance of hydrocarbon oils in combination with steel surfaces [1,2]. Predicting additive effectiveness; however, is very complex. The additive performance is well known to be dependent on the entire tribosystem, meaning that the effect of base fluids, surface materials, and operating conditions all must be considered [3]. Our understanding of tribo-improving additives is still evolving, and significant clues on the mechanisms of these are still being uncovered [4,5]. Boundary lubrication (BL) additives are generally categorized as extreme pressure (EP) or anti-wear (AW) additives and form boundary films by adsorption on metal surfaces followed by chemical reaction [6]. AW films form a protective film that shields the underlying surface, whereas sacrificial EP films can actually increase wear, but are used to prevent catastrophic seizure [3]. The mechanism of film formation implies that two rates must be controlled: first, the rate of adsorption onto the surface; and second, the rate of formation of the reacted films, which must be balanced in accordance to the operational conditions faced [7]. To correctly balance both rates in practical applications is a challenging task that is further complicated by interference between different additives, as well as the influence of the base fluid quality. Therefore, arriving at successful lubricant formulations (base oil and additive combination) has been largely empirical in nature. Synthetic base fluids are increasingly being relied upon to meet requirements of low volatility and thermal stability; however, the additive technology developed for hydrocarbon base fluids is not necessarily applicable due to the significant difference in chemical structure between the traditional hydrocarbon base fluids and the novel advanced synthetic fluids. In many cases, the conventional additives are not miscible [8; 9]. In other cases, if the base fluid is very polar, miscibility may become excessive, resulting in a low affinity for the metal surfaces it should protect [10]. Room temperature ionic liquids (RTILs) are a class of advanced synthetic fluids that are highly polar as the fluids consist entirely of cations and anions [11; 12; 13; 14]. These fluids were first suggested as advanced lubricating fluids in 2001 in Reference [15]. As described in Reference [16], they have been intensely researched in many fields, including tribology, over the past two decades. The ionic nature means that RTILs readily adsorb on metal surfaces and under tribological conditions can form boundary films, even as neat fluids [17; 18]. However, as mentioned above, optimum reactivity depends on the entire tribosystem, and even though RTIL properties can be tailored by modifying their chemical structure, this is inefficient compared to the approach of base fluid-additive systems. Additives provide the option to fine tune lubricant performance to meet the demands of specific operational conditions without redesigning the base fluid. There have been many attempts with both conventional and unconventional additives for RTILs [19; 20; 21; 22; 23], but inherently high reactivity remains problematic. However, in our recent work, hydrocarbon-mimicking ionic liquids have been designed for use as base fluids to reduce the inherent reactivity and enable compatibility with existing additives [24].