A lubricating oil composition is provided, which comprises a lubricating base oil and based on the whole weight of the lubricating oil composition, (A) 0.5 wt % to 10 wt % of a metal salicylate having a total base number of from 100 mg-KOH/g to 195 mg-KOH/g, (B) 0.1 wt % to 10 wt % of a diarylamine compound, (C) 0.1 wt % to 10 wt % of a hindered phenol compound, and (D) 1 wt % to 10 wt % of a polyalkenylsuccinimide and/or a boron-containing polyalkenylsuccinimide. Also provided is a lubricating oil composition which further comprises 0.1 wt % to 10 wt % of a metal phenate having a total base number of from 100 mg-KOH/g to 300 mg-KOH/g as a component (E) in addition to the above components (A) to (D). The lubricating oil composition is excellent in all properties of detergency, NO.sub.x oxidation resistance and thermal oxidation resistance and suitable as a long-life engine oil for gas engine heat pumps.
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