Lubricant Compositions Containing Low Mono-Alkyl-Substituted Diphenylamine Antioxidant for Use in Soot Control Applications

A method to reduce sooted viscosity in an internal combustion engine comprising lubricating an internal combustion engine with a lubricating oil composition comprising:

• 1) 50 mass % or more of one or more base oils;
• 2) diphenylamine antioxidant comprising:
• at least 85 mass % of di(C8-9 substituted phenyl)amine and from 0 to 15 mass % (such as 1 to 10 mass %, such as 0 to 8 mass %, such as 0 to 5 mass %) of mono(C8-9 substituted phenyl)amine, and from 0 to 2 mass % of unsubstituted diphenylamine, based upon the weight of the unsubstituted, mono substituted- and disubstituted-diphenylamines;
• 3) detergent;
• 4) dispersant;
• and
• 5) optional styrenic block copolymer viscosity modifier,
• wherein the lubricating oil composition, when containing 12 mass % carbon black (based upon the weight of the lubricating oil composition), has a dynamic viscosity (Pas) at a shear rate of 1.3 (l/s) at least 10% lower (such as at least 15% lower, such as at least 20% lower, such as at least 25% lower) than the dynamic viscosity at a shear rate of 1.3 (l/s) of the same lubricating oil composition except that the C8-9 diphenylamine antioxidant has been replaced with the same amount of diphenylamine antioxidant having:
• 1) about 68 mass % di-C9-alkyl diphenylamine,
• 2) about 29 mass % of mono-C9-alkyl diphenylamine, and
• 3) about 3 mass % of tri-C9-alkyl diphenylamine, based on the weight of 1), 2) and 3), as determined by UPLC in the Experimental section.