Extended-reach-drilling (ERD) wells are expensive and challenging; however, in special situations, compared with conventional drilling, ERD wells are more environmentally friendly and cost-effective. Application of drilling fluids with good lubrication for ERD is one of the most important methods to facilitate longer total depth (TD) of the wells. To better simulate the elevated-temperature environment in the borehole, this study proposes a method to perform tribological studies of drilling fluids at temperatures higher than 100C by conducting experiments in a high-chamber-pressure environment, which can suppress the evaporation of the drilling fluid at high temperatures. Two lubricant additives were studied, and the results showed that, for the drilling fluid at elevated temperatures, a prototype additive (Additive A) reduced the coefficient of friction (COF) significantly by 44.8%, whereas a commercial additive (Additive B) caused only a slight reduction of the COF by 4%. After the tribological experiments, the wear mechanisms of the additives and abrasive particles were investigated with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).