Water-in-oil (W/O) emulsions are very common in oil field operations and are formed as a result of energy input from turbulence caused by the flow in the production pipelines, pumps, and valves. Understanding emulsion rheological behavior is crucial to deal with flow assurance issues. This paper presents and discusses a series of rheological experiments carried out with synthetic emulsions formulated with 126 Brazilian crude oils with American Petroleum Institute (API) gravity ranging from 13 to 35 degrees. This rheological study includes viscosity dependence upon the shear rate, temperature, and water volume fraction: The results show that crude oils with similar API gravity and viscosity can generate emulsions with very different viscosities (8-50 mPa s at 50 degrees C around 25 degrees API gravity, for example) and different maximum water content limits. Besides, W/O emulsions that are prepared with either light (API gravity of >35 degrees) or heavy (API gravity of <13 degrees) crude oils are the emulsions observed to be the more difficult to stabilize, particularly the high-water-cut emulsions. Also, a large amount of data show that intermediate API gravity crude oils can incorporate up to 70% water volume fraction and show the highest relative viscosity. As a general trend, W/O emulsions show a typical Newtonian behavior at temperatures above the wax appearance temperature and at low water cuts. The rheological study shows that the temperature, shear rate, water volume fraction, and API gravity have important impacts on the viscosity of W/O emulsions.