Qualitative discrimination between API GL performance levels of manual transmission fluids by comparing their EP properties determined in a new four-ball scuffing test

► We have developed a new four-ball test method, performed under scuffing conditions. ► We tested manual transmission fluids (MTFs) of various API GL performance levels. ► Qualitative assessment of MTFs is performed by analysing friction torque curves. ► The method discriminates between MTFs of API levels: GL-1, GL-3, GL-4/5, and GL-5+. ► We explain surface phenomena in different phases of the scuffing process for MTFs.