Patent ID: 7772171

Claim:
A method of lubricating an internal combustion engine equipped with a catalyst-containing exhaust-gas aftertreatment device, the method comprising supplying to said engine a lubricating oil composition comprising: (a) a base oil; and (b) a zinc salt of a mixture of phosphorus-containing compounds represented by the formula wherein in formula (I), X 1 and X 2 are independently S or O, and R 1 and R 2 are independently hydrocarbyl groups, the average total number of carbon atoms in R 1 plus R 2 for the mixture of phosphorus-containing compounds being at least about 9.5; wherein R 1 and R 2 are characterized in that (i) about 4 to about 30 weight percent of such groups contain 2 to 4 carbon atoms and (ii) about 70 to about 96 weight percent such groups contain 5 to 12 carbon atoms; and wherein, in less than 8 mole percent of the molecules of formula (I) in the mixture of phosphorus-containing compounds, each of R 1 and R 2 contain 2 to 4 carbon atoms and in greater than 11 mole percent of the molecules of formula (I) in said mixture R 1 has 2 to 4 carbon atoms and R 2 has 5 to 12 carbon atoms; and wherein, within formula (I), the average total number of hydrogen atoms in R 1 and R 2 on carbon atoms located beta to the O atoms is at least 7.25; the lubricating oil composition being characterized by a phosphorus concentration of up to about 0.12 percent by weight.