A lubricating oil composition comprising:
(1) 98.6 to 53% by weight of a base oil having a kinematic viscosity of 3 to 20 centistokes as determined at 100°C;
(2) 0.2 to 5% by weight of a sulfurized oxymetal organo-phosphorodithioate represented by the formula (I):
and/or sulfurized oxymetal dithiocarbamate represented by the formula (II):
(3) 0.1 to 7% by weight of at least one zinc dithiophosphate represented by the formula (III):
(4) 0.1 to 20% by weight of a calcium alkylbenzenesulfonate and/or a calcium petroleumsulfonate; and (5) 1 to 15% by weight of alkenylsuccinic acid imide and/or boron derivatives thereof. The present lubricating oil composition is superior in reducing the mechanical friction loss or four-cycle engines, particularly small-sized high power four-cycle engines when used as an engine oil.