As part of a study to determine the lubricant factors affecting fatigue of rolling contact bearings, the effects of base oil composition and viscosity for additive blends on the fatigue life of CVD 52100 steel balls were evaluated with a 4-ball fatigue machine. Using pure hydrocarbons as base stocks, aromatic hydrocarbons were found to not only give longer lives than paraffins or naphthenes but also exhibit extremes in additive response. Using different distillation cuts to cover the viscosity range from 2.3 to 24 mm2/s, life tended to increase with increasing viscosity for each of four series of mineral oils whereas the addition of antiwear and EP additives generally reduced life. In contrast, increasing the viscosity by reducing the test temperature increased life for most additive blends. The hydraulic wedge mechanism for fatigue, modified to include surface coating formation effects, seems to account for the observed results.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference held in Miami Beach, Florida, October 21–23, 1975