Viscosity index improver and lubricating oil composition

A viscosity-index improving agent comprising:

• a (co) polymer (A) containing, as an essential constituent monomer, a polyolefin-based monomer (a) represented by the following formula (1);
• an ester oil (B1) having a kinematic viscosity at 100° C. of 1.00 to 2.50 mm2/s;
• and
• an ester oil (B2) having a kinematic viscosity at 100° C. of 2.51 to 5.00 mm2/s:
• wherein a weight ratio {(B1)/(B2)} of the ester oil (B1) to the ester oil (B2) in the viscosity-index improving agent is 25/75 to 75/25,
• wherein R1 is a hydrogen atom or a methyl group;
• —X1— is a group represented by —O—, —O(AO)m—, or —NH—, A is a C2-C4 alkylene group, m is an integer of 1 to 10, and each A may be the same or different when m is 2 or more;
• R2 is a residue after removal of one hydrogen atom from a hydrocarbon polymer containing a 1,2-butylene group as a structural unit;
• and p represents a number of 0 or 1, and
• wherein a difference ((B2)−(B1)) in kinematic viscosity at 100° C. between the ester oil (B1) and the ester oil (B2) in the viscosity-index improving agent is 0.88 to 1.3 mm2/s
• wherein a ratio {(A)/((B1)+(B2))} of the weight of the (co)polymer (A) to the total weight of the ester oil (B1) and the ester oil (B2) in the viscosity-index improving agent is 15/85 to 43/57, and
• wherein, when the viscosity-index improving agent is added to a lubricating oil composition comprising 76.55 wt. % to 86.04 wt. % of a hydrocarbon oil, the lubricating oil composition has a friction coefficient of 0.112 to 0.123,
• wherein the friction coefficient is measured using a high-frequency reciprocating rig with a test steel ball made of SUJ2 and having a diameter of 6 mm and a test disk made of SUJ2 under the following conditions:
• measurement temperature:
• 80° C.;
• load:
• 3.92 N (400 g);
• stroke:
• 1000 μm;
• and
• frequency:
• 20 Hz.