The viscosity of oil after it has been used in an engine over a period of time may change from its initial value because the high temperature inside the engine can cause the oil to break down. An experiment was conducted to compare the effect on oil viscosity of two different engines. Various samples of the same type of oil with a constant viscosity were used, some in engine I and some in engine 2 and the engines were run under identical operating conditions. The resulting values of the oil viscosities after having been used in the engines are given in DS 9.6.6. Is there any evidence that the engines have different effects on the oil viscosity?