This study deals with the synthesis of estolides from high-oleic sunflower oil oleins, and its potential use as viscosity modifiers for biolubricant applications. Synthesis reactions were monitored for 24 h. Estolide molecular weight increased continuously with reaction time. Dynamic viscosities and densities of the different estolides were measured in a temperature range comprised between 10 and 120 °C. Maximum viscosities and estolide molecular weight were obtained after 12 h of reaction. However, the largest viscosity increments were observed within the first 3 hours of processing, due to a dramatic increase in the molecular weight of the estolides.