Lubricating oil composition for transmission

A lubricating oil composition for transmissions, comprising (A) a lubricating base oil with a kinematic viscosity at 100° C. adjusted to 1.5 to 6 mm2/s, composed of (A1) a lubricating base oil with a kinematic viscosity at 100° C. of 1.5 mm2/s or higher and lower than 7 mm2/s or (A1) the lubricating oil and (A2) a lubricating base oil with a kinematic viscosity at 100° C. of 7 to 50 mm2/s, blended with (B) a poly(meth)acrylate-based additive containing a structural unit represented by formula (1) below, so that the composition has a kinematic viscosity at 100° C. of 3 to 8 mm2/s and a viscosity index of 95 to 200, the composition fulfilling at least one requirement selected from the following [I] to [III]:

• [I] Component (A) is a lubricating base oil having a kinematic viscosity at 100° C. adjusted to 1.5 to 4.5 mm2/s and Component (B) is (B 1) a poly(meth)acrylate-based additive containing a structural unit of formula (1) wherein R2 is a straight-chain or branched hydrocarbon group having 16 to 30 carbon atoms;
• [II] Component (A) is a lubricating base oil with a kinematic viscosity at 100° C. adjusted to 1.5 to 6 mm2/s, composed of 70 to 97 percent by mass of Component (A1) and 3 to 30 percent by mass of Component (A2), and Component (B) is (B2) a poly(meth)acrylate-based additive substantially not containing a structural unit of formula (1) wherein R2 is a hydrocarbon group having 20 or more carbon atoms;
• and
• [III] the kinematic viscosity at 100° C. (Vc) of the composition is from 4.5 to 8 mm2/s, and the ratio of the kinematic viscosity at 100° C. (Vb) of Component (A) to (Vc) (=Vb/Vc) is 0.70 or greater,
• wherein R1 is hydrogen or methyl, R2 is a hydrocarbon group having 1 to 30 carbon atoms or a group represented by —(R)a-E wherein R is an alkylene group having 1 to 30 carbon atoms, E is an amine residue or a heterocyclic residue, each having 1 or 2 nitrogen atoms and 0 to 2 oxygen atoms, and a is an integer of 0 or 1.