The formation of water-in-oil (W/O) emulsions during extraction of petroleum is a major problem, because these emulsions increase the oil's viscosity and thus significantly lower the production rates. In these situations, chemical products called emulsion viscosity reducers (EVRs) can be injected during crude oil production. The development of EVRs consisting of oil-in-water (O/W) microemulsions can attract a good deal of attention due to the possibility of reducing costs and increasing environmental friendliness (they are mostly composed of water instead of large volumes of organic solvents). This article describes the preparation of O/W microemulsions, using the solvents kerosene, Solbrax or xylene as components of the oil phase and three commercial nonionic polymer surfactants based on poly(ethylene oxide) as the aqueous phase. For this purpose, first ternary phase diagrams (water/oil/surfactant) were constructed to determine the optimal concentrations of these components to prepare the O/W microemulsions, selecting 16 wt% surfactant, 4 wt% oil phase and 80 wt% aqueous phase as the common composition of all the systems studied. The efficiency of the microemulsion systems was investigated by rheological tests in synthetic water/oil emulsions prepared from a heavy crude oil (degrees API 20). The microemulsion systems were tested regarding their interfacial activity by analyzing the interfacial tension and the average size of the water droplets dispersed in the crude oil, before and after adding the systems developed. The results showed that the performance of the O/W microemulsions did not depend on the type of oil phase, but that the insertion of an oil phase (i.e., the formation of an O/W microemulsion) improved the performance of the surfactant in the medium compared to the efficiency of the aqueous surfactant solutions.