Patent ID: 8377856

Claim:
A method for maintaining a reduction in total base number (TBN) of less than about 4.0 mg KOH/g for a lubricant for a medium speed diesel engine for an extended period of time of greater than about 184 days of use of the lubricant in service in the engine comprising: supplying as the lubricant, a lubricant composition comprising (1) a detergent additive consisting essentially of (a) a sulfonate detergent having a TBN ranging from about 20 to about 100 mg KOH/g and (b) a phenate detergent having a TBN ranging from about 200 to about 350 mg KOH/g, wherein the phenate detergent is present in an amount that is greater than 15 percent by weight of the total weight of detergents (a) and (b) in the lubricant composition and the total weight of detergents (a) and (b) in the lubricant composition ranges from 0.5 to 5.0 wt. % based on a total weight of the lubricant composition; and (2) an antioxidant selected from the group consisting of hindered phenolic derivatives of C 3 to C 6 acids and C 7 to C 9 esters of hindered phenolic derivatives of C 3 to C 6 acids, wherein the lubricant composition is substantially devoid of zinc dialkydithiophosphate compounds, and wherein the lubricant composition has a sulfated ash content of less than 1.0 wt. % and an initial TBN of ranging from about 7.5 to about 8.4 mg KOH/g; and operating the engine on the lubricant composition for the extended period of time of greater than about 184 days between lubricant drain intervals, wherein the TBN reduction is less than about 4.0 mg KOH/g after about 184 days.