A lubricating oil composition contaminated with at least 0.3 wt% of a biodiesel fuel, based on the total weight of the lubricating oil composition, wherein the lubricating oil composition comprises: a. at least 40 wt% of base oil of lubricating viscosity by weight of the lubricating oil composition; and b. a Mannich condensation product, wherein the Mannich condensation product is of the formula 7 wherein each R is independently -CHR'-, wherein R' is branched or linear alkyl having one to 10 carbon atoms, cycloalkyl having from 3 carbon atoms to 10 carbon atoms, aryl having from 6 carbon atoms to 10 carbon atoms, alkaryl having from 7 carbon atoms to 20 carbon atoms, or aralkyl having from 7 carbon atoms to 20 carbon atoms, R 1 is a polyisobutyl group derived from polyisobutene containing at least 70 wt. % methylvinylidene isomer and having a number average molecular weight in the range of 400 to 2,500; X is hydrogen, an alkali metal ion, or alkyl having one carbon atom to 6 carbon atoms; W is -[CHR"]- m, wherein each R" is independently H, alkyl having one carbon atom to 15 carbon atoms, or a substituted-alkyl having one carbon atom to 10 carbon atoms and one or more substituents selected from the group consisting of amino, amido, benzyl, carboxyl, hydroxyl, hydroxyphenyl, imidazolyl, imino, phenyl, sulfide, or thiol; and m is an integer from one to 4; Y is hydrogen, alkyl having one carbon atom to 10 carbon atoms, -CHR'OH, wherein R' is as defined above, or of formula 8: wherein Y' is -CHR'OH, wherein R' is as defined above; and R, X, and W are as defined above; Z is hydroxyl, a hydroxyphenyl group of the formula 9: or of formula 10: wherein R, R 1, Y', X, and W are as defined above, and n is an integer from 0 to 20, with the proviso that when n = 0, Z must be of formula 10 as defined above. a. at least 40 wt% of base oil of lubricating viscosity by weight of the lubricating oil composition; and b. a Mannich condensation product, wherein the Mannich condensation product is of the formula 7 wherein each R is independently -CHR'-, wherein R' is branched or linear alkyl having one to 10 carbon atoms, cycloalkyl having from 3 carbon atoms to 10 carbon atoms, aryl having from 6 carbon atoms to 10 carbon atoms, alkaryl having from 7 carbon atoms to 20 carbon atoms, or aralkyl having from 7 carbon atoms to 20 carbon atoms, R 1 is a polyisobutyl group derived from polyisobutene containing at least 70 wt. % methylvinylidene isomer and having a number average molecular weight in the range of 400 to 2,500; X is hydrogen, an alkali metal ion, or alkyl having one carbon atom to 6 carbon atoms; W is -[CHR"]- m, wherein each R" is independently H, alkyl having one carbon atom to 15 carbon atoms, or a substituted-alkyl having one carbon atom to 10 carbon atoms and one or more substituents selected from the group consisting of amino, amido, benzyl, carboxyl, hydroxyl, hydroxyphenyl, imidazolyl, imino, phenyl, sulfide, or thiol; and m is an integer from one to 4; Y is hydrogen, alkyl having one carbon atom to 10 carbon atoms, -CHR'OH, wherein R' is as defined above, or of formula 8: wherein Y' is -CHR'OH, wherein R' is as defined above; and R, X, and W are as defined above; Z is hydroxyl, a hydroxyphenyl group of the formula 9: or of formula 10: wherein R, R 1, Y', X, and W are as defined above, and n is an integer from 0 to 20, with the proviso that when n = 0, Z must be of formula 10 as defined above. wherein each R is independently -CHR'-, wherein R' is branched or linear alkyl having one to 10 carbon atoms, cycloalkyl having from 3 carbon atoms to 10 carbon atoms, aryl having from 6 carbon atoms to 10 carbon atoms, alkaryl having from 7 carbon atoms to 20 carbon atoms, or aralkyl having from 7 carbon atoms to 20 carbon atoms, R 1 is a polyisobutyl group derived from polyisobutene containing at least 70 wt. % methylvinylidene isomer and having a number average molecular weight in the range of 400 to 2,500; X is hydrogen, an alkali metal ion, or alkyl having one carbon atom to 6 carbon atoms; W is -[CHR"]- m, wherein each R" is independently H, alkyl having one carbon atom to 15 carbon atoms, or a substituted-alkyl having one carbon atom to 10 carbon atoms and one or more substituents selected from the group consisting of amino, amido, benzyl, carboxyl, hydroxyl, hydroxyphenyl, imidazolyl, imino, phenyl, sulfide, or thiol; and m is an integer from one to 4; Y is hydrogen, alkyl having one carbon atom to 10 carbon atoms, -CHR'OH, wherein R' is as defined above, or of formula 8: wherein Y' is -CHR'OH, wherein R' is as defined above; and R, X, and W are as defined above; Z is hydroxyl, a hydroxyphenyl group of the formula 9: or of formula 10: wherein R, R 1, Y', X, and W are as defined above, and n is an integer from 0 to 20, with the proviso that when n = 0, Z must be of formula 10 as defined above.