Here I present Rheology of Engine Oils that helps to provide the present behavior of the oil as well as its behavior after weeks, months, years and even decades. Functioning of engine oil will improve and are cost effective, therefore, no need to change engine oil frequently. The desirable properties of engine oils cannot be achieved economically by physical and chemical processing of crude oils alone. Therefore, engine oils are produced by blending mineral base oils and chemical additives for achieving the desired properties. Viscosity Index improvers are used in multigrade engine oils for improving viscometric and rheological properties. Different Viscosity Index improvers modify the rate of change of viscosity with temperature, they cause a minimal increase in engine oil viscosity at low temperatures but cause considerable increase at high temperatures.