Lubricant composition for oxidation-stable automotive transmission

A method for improving the oxidation stability of a lubricant composition comprising at least one base oil, the method comprising adding a phosphite polymer in combination with a boron-containing dispersant and two antioxidants into the lubricant composition, said phosphite polymer replying to formula (I):

• where:
• each of R1, R2, R3 and R4 can each be independently selected from among the following groups:
• C1-C20 alkyl, C3-C22 alkenyl, C6-C40 cycloalkyl, C7-C40 cycloalkenyl, C1-20 methoxy alkyl glycol ethers and Y—OH (acting as terminal group);
• Y is selected from among the following groups:
• C2-C40 alkylene, C2-C40 alkyl lactone, —R7—N(R8)—R9— where R7, R8 and R9 are each independently selected from among hydrogen, C1-C20 alkyl, C3-C22 alkenyl, C6-C40 cycloalkyl, C7-C40 cycloalkenyl, C1-20 methoxy alkyl glycol ethers;
• m is an integer ranging from 2 to 100;
• and
• n is an integer ranging from 1 to 1000,
• said two antioxidants being selected from among amine antioxidants, phenolic antioxidants substituted by one or more linear or branched alkyl groups having 1 to 30 carbon atoms, and mixtures thereof,
• wherein the lubricant composition comprises 70 to 99% by weight of one or more base oils, 0.01 to 10% by weight of phosphite polymer, 0.01 to 5% by weight of boron-containing dispersant(s), 0.01 to 5% by weight of the mixture of at least one amine antioxidant and at least one phenolic antioxidant substituted by one or more linear or branched C1-C30 alkyl groups, relative to the total weight of the lubricant composition.