In the field of lubricating oil used in applications such as internal combustion engine and automatic transmission, in recent years, from such points of view as efficient use of resources, decrease in waste oil and cost reduction for users of lubricating oil, requirements have increased regarding lubricating oils to have longer drain intervals.
Thus, in prior art lubricating oils, in response to the above-mentioned requirements, improvement of oxidation stability of lubricating oil is intended, by taking a mineral oil in which the aromatic content has been decreased by high-level refining (highly refined mineral oil) serving as the base oil, and including therein large amounts of a reaction chain terminator, such as a phenol type or an amine type antioxidant, and a sulfur-containing compound having peroxide decomposing ability, such as, zinc dithiophosphate (ZDTP) or molybdenum dithiocarbamate (MoDTC), or the like (for instance, refer to Patent Reference 1).
However, there are limits to maintaining the oxidation stability by a increase of the reaction chain terminator, and in addition, there is the problem that deterioration products of the reaction chain terminator become insoluble. In addition, a sulfur-containing compounds such as ZDTP may per se oxidize or thermally decompose and become the cause of acidic substance generation, such as of sulfuric acid, therefore, in lubricating oils for internal combustion engine, or the like, where metallic detergent and ashless dispersant are mixed in general, it becomes a cause that triggers a decrease in base number, which is an indicator for acid neutralization property, and a decrease in high temperature detergency.
On the other hand, in general, decreasing the quantity of metallic detergent mixed in the lubricating oil is considered to be necessary in order to prevent ash deposition onto pistons of internal combustion engines, catalysts, such as, three-way catalysts, oxidation catalysts and NOx occlusion reduction type catalysts, or emission gas refining devices, such as, DPF; however, simply decreasing the quantity of mixed metallic detergent compromises acid neutralization properties and high temperature detergency.
Furthermore, in regard to the above-mentioned catalysts, in particular the NOx occlusion reduction type catalyst, in order to decrease poisoning due to sulfur, studies on reducing sulfur content in fuels (for instance, gas oil having a sulfur content of 50 mass ppm or less, gasoline having a sulfur content of 10 mass ppm or less, and the like) are proceeding rapidly, and the effects thereof are anticipated, however, in an internal combustion engine using such low sulfur fuels, the influence on the catalysts exerted by the sulfur content in the lubricating oil become relatively important. Consequently, a further increase in the amounts of sulfur-containing compounds in the lubricating oil is not desirable, and decreasing the sulfur content in the lubricating oil is essential.
Thus, studies were made to solve the above-mentioned problematic points in conventional lubricating oils and achieve sufficiently longer drain intervals. For instance, disclosed are, in Patent Reference 2, a lubricating oil composition in which a specific phosphorus compound has been mixed, in Patent Reference 3, a lubricating oil composition in which the content of ZDTP has been decreased and at the same time salicylate and sulfonate and a phosphorus-containing anti-wear agent are used in combination, and, in Patent Reference 4, a lubricating oil composition in which a specific phosphorus compound and a sulfur-containing organic molybdenum complex are used in combination, respectively.    [Patent Reference 1] Japanese Patent Application Laid-open No. H8-302378    [Patent Reference 2] Japanese Patent Application Laid-open No. 2002-294271    [Patent Reference 3] Japanese Patent Application Laid-open No. 2003-277781    [Patent Reference 4] Japanese Patent Application Laid-open No. 2004-83891
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