Lubricating oil composition for engine made of aluminum alloy and lubrication method

A lubricating oil composition, comprising:

• a base oil;
• a first succinimide compound;
• a second succinimide compound;
• and
• a thioheterocyclic compound represented by formula (I):
• (R1)k—(S)m-AS-(S)n—(R2)l (I)
• where
• AS is a 1,3,4-thiadiazole ring;
• (S)m and (S)n are bonded to the 1,3,4-thiadiazole ring at the 2-position and the 5-position, respectively;
• each of R1 and R2 independently represents a hydrogen atom, an amino group, and a C1 to C50 group selected from the group consisting of an alkyl group, a cycloalkyl group, an alkenyl group, a cycloalkenyl group, and an aryl group, the C1 to C50 group optionally including at least one oxygen, nitrogen, or sulfur heteroatom;
• each of k, l, m, and n is an integer of 1 to 5;
• wherein:
• the first succinimide compound comprises at least one of an alkenylsuccinimide and an alkylsuccinimide;
• the second succinimide compound comprises at least one of a modified alkenylsuccinimide and a modified alkylsuccinimide;
• the second succinimide compound is modified with at least one of a boron compound and an organic acid;
• a boron content of the composition is 0.06 to 0.3 mass % based on a total mass of the composition;
• a ratio by mass of boron to nitrogen (B/N) in the composition is 0.25 to 1.0;
• a sulfur content of the composition is 0.10 mass % to 1.00 mass % based on a total mass of the composition;
• and
• a phosphorus content represented by P in mass % and a sulfated ash content represented by M in mass %, based on the total mass of the composition, satisfy any of conditions A to C:
• condition A:
• P<0.03, and M<0.3;
• condition B:
• P<0.03, and 0.3≦M≦0.6;
• and
• condition C:
• 0.03≦P≦0.06, and M<0.3.