Conventionally, lubricating oil has been used in an internal combustion engine, a transmission or other mechanical devices to allow the smooth operation thereof. In particular, a lubricating oil (engine oil) for an internal combustion engine is required to exhibit performances of higher levels because internal combustion engines have been improved in performances, enhanced in output and used under severe working conditions. Therefore, indispensably the engine oil maintains the viscosity at high temperatures. In order to meet such demands, conventional engine oils have contained various additives such as an antiwear agent, a metallic detergent, an ashless dispersant, and an anti-oxidant (for example, see Patent Literatures 1 to 3 below).
Furthermore, recently the expectations of the fuel saving performance of the lubricating oil have been higher and higher, and thus applications of a high viscosity index base oil or various friction modifiers has been studied (for example, see Patent Literature 4 below).
By the way, a system for generating electric power utilizing an internal combustion engine as a means for providing driving force has existed through the ages. However, no concern has been made for the fuel economy provided by the lubricating oil used in this system so far.
However, some automobiles such as hybrid cars have been equipped with a motor used to provide part of driving force and the engine has been used to drive the motor when used as a generator or drive both the motor and generator rather than to provide the automobiles with driving force.