In response to the challenges posed by friction and wear of tribo-components exposed to boundary and mixed lubrication conditions, there is increasing interest in the development of polymer-based, self-lubricating composites. This investigation addressed the effect of various fillers (i.e. short carbon fibers (SCF), short glass fibers (SGF), ZnS sub-micrometer particles and combinations of respective fibers with ZnS particles), on the friction and wear of polyimide (PI) when lubricated by poly-alpha-olefins (PAO). A sliding plate-on-ring geometry, using GCr15 steel rings, was used. It was demonstrated that the reinforcing fibers greatly improved the composites’ wear resistance. SGF was observed to be more effective than SCF in respect to friction- and wear-reduction. Moreover, an obvious synergism between SGF and ZnS was identified. The hybrid composite ZnS/SGF/PI exhibited excellent tribological performance under oil lubrication. With respect to the hybrid formulation, the glass fibers imparted their high abrasion resistance to the composite; while ZnS led to complex tribo-chemical reactions. A stable tribofilm consisting of transferred materials and various tribo-chemical products improved the boundary lubrication capability of the sliding system.