Viscosity modifier of lubricating oil for power transmission system and lubricating oil composition for power transmission system

A lubricating oil composition for a power transmission system, comprising:

• a mineral oil type lubricant base oil (A) having the following properties (A0-1) to (A0-3):
• (A0-1) a kinematic viscosity at 100° C. in the range of 2 to 10 mm2/s,
• (A0-2) a viscosity index not less than 90, and
• (A0-3) a pour point not higher than −20° C.,
• and
• 0.1 to 10% by weight of an ethylene/propylene copolymer (B) having the following properties (B1) to (B4):
• (B1) an ethylene content in the range of 78.0 to 85% by mol.,
• (B2) an intrinsic viscosity [η] in the range of 0.2 to 0.75 dl/g,
• (B3) Mw/Mn not more than 2.4, and
• (B4) a melting point, as measured by DSC, not higher than 60° C., or no melting point is observed.