Studies of transitions between low and high friction and wear in the four-ball machine are extended and confirmed with lubricants consisting of noncyclic hydrocarbons and dilute solutions of stearic acid in cetane and squalane. These show transition temperatures that are the same for AISI 4140 and 52100 steels and that increase with increasing ratio of speed to load. Hydrocarbons with cyclic structures show similar performance with 4140 steel and, at high speed-load ratios, with 52100 steel. However, with the 52100 steel at low speed-load ratios there is a speed-load ratio independent transition temperature. Neat stearic acid with 52100 steel shows similar performance to the cyclic hydrocarbons with the speed-load ratio independent transition temperature agreeing with pin-on-disk machine results on other steels in the literature. These and literature results are rationalized by a mechanism involving viscous trapping of lubricant between interacting load-supporting asperities.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Washington, D.C., October 13–16, 1964.