Lowering the viscosity of engine oil is effective in reducing the fluid friction. However, it decreases the oil film thickness, and causes the increase in the wear of engine parts. Through the engine wear tests using an radioisotope tracer technique, it was clarified that an HTHS viscosity of 2.6 mPa„s was the lower limit to prevent the increasing wear. It was also found that the influence of the lowering viscosity on the wear of piston rings was larger than that on the wear of the cams and connecting rod bearings.

Addition of friction modifiers is effective in reducing the friction under boundary lubricating conditions. MoDTC was determined as an excellent friction modifier, after evaluating its effects on the friction of valve train and on the fuel consumption of engine. The effect of MoDTC on the friction reduction was considered to be caused by both of the MoS₂ film formed and the rubbing surface smoothness that decreased the metal contact.

From the results of these investigations, it was concluded that the points of designing low friction engine oil were lowering the HTHS viscosity to 2.6 mPa„s and the addition of MoDTC. Based on this guideline, a low friction engine oil has been developed, and it contributes to the improving fuel consumption of automobiles.
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