Lubricating oil composition for transmissions

A lubricating oil composition for transmissions which comprises a lubricating base oil comprising (A) from 60 to 94 percent by mass, based on the total amount of the base oil, of a lubricating base oil so adjusted to have a kinematic viscosity at 100° C. of from 1.5 to 5 mm2/s, a % CN of from 10 to 60 and a % CA of 1 or less, (B) from 5 to 25 percent by mass, based on the total amount of the base oil, of a mineral lubricating base oil having a kinematic viscosity at 100° C. of from 10 to 50 mm2/s and a sulfur content of from 0.3 to 1 percent by mass and (C) from 1 to 15 percent by mass, based on the total amount of the base oil, of a synthetic oil composed of carbon and hydrogen and having a number average molecular weight of from 2,000 to 20,000, and (D) from 0.05 to 2 percent by mass, based on the total amount of the composition, of an extreme pressure additive comprising a phosphorus-based extreme pressure additive, a sulfur-based extreme pressure additive and/or a phosphorus-sulfur-based extreme pressure additive, wherein in the composition, the phosphorus content (P) is from 0.01 to 0.05 percent by mass, the total sulfur content (S) is from 0.05 to 0.3 percent by mass, and the P/S ratio is from 0.10 to 0.40.

A method of improving the fatigue life properties of a lubricating oil composition for transmissions which comprises a lubricating base oil comprising (A) from 60 to 95 percent by mass, based on the total amount of the base oil, of a lubricating base oil so adjusted to have a kinematic viscosity at 100° C. of from 1.5 to 5 mm2/s and a % CN of from 10 to 60 and (B) from 5 to 40 percent by mass, based on the total amount of the base oil, of a mineral lubricating base oil having a kinematic viscosity at 100° C. of from 10 to 50 mm2/s and a sulfur content of from 0.3 to 1 percent by mass, and (D) from 0.05 to 2 percent by mass, based on the total amount of the composition, of an extreme pressure additive comprising a phosphorus based extreme pressure additive, a sulfur-based extreme pressure additive and/or a phosphorus-sulfur-based extreme pressure additive, wherein in the composition, the phosphorus content (P) is from 0.01 to 0.05 percent by mass, the total sulfur content (S) is from 0.05 to 0.3 percent by mass, and the P/S ratio is from 0.10 to 0.40.

A method of improving the fatigue life properties of a lubricating oil composition for transmissions which comprises a lubricating base oil comprising (A) from 60 to 94 percent by mass, based on the total amount of the base oil, of a lubricating base oil so adjusted to have a kinematic viscosity at 100° C. of from 1.5 to 5 mm2/s, a % CN of from 10 to 60 and a % CA of 1 or less, (B) from 5 to 25 percent by mass, based on the total amount of the base oil, of a mineral lubricating base oil having a kinematic viscosity at 100° C. of from 10 to 50 mm2/s and a sulfur content of from 0.3 to 1 percent by mass and (C) from 1 to 15 percent by mass, based on the total amount of the base oil, of a synthetic oil composed of carbon and hydrogen and having a number average molecular weight of from 2,000 to 20,000, and (D) from 0.05 to 2 percent by mass, based on the total amount of the composition, of an extreme pressure additive comprising a phosphorus-based extreme pressure additive, a sulfur-based extreme pressure additive and/or a phosphorus-sulfur-based extreme pressure additive, wherein in the composition, the phosphorus content (P) is from 0.01 to 0.05 percent by mass, the total sulfur content (S) is from 0.05 to 0.3 percent by mass, and the P/S ratio is from 0.10 to 0.40.