Use of a lubricating oil having a kinematic viscosity at 100°C of 25 mm 2 /s or less comprising: (a) a base oil comprising a bimodal blend of two different base oils, i. the first base oil being one or more oils selected from the group consisting of Group I base oils having a kinematic viscosity at 100°C of from 2 to less than 12 mm 2 /s and ii. a second base oil being one or more oils selected from Group IV base oils having a kinematic viscosity at 100°C of at least 38 mm 2 /s, the difference in kinematic viscosity between the first and second base oils being at least 30 mm 2 /s, and (b) one or more detergents selected from the group consisting of alkali and/or alkaline earth metal sulfonate, phenate, salicylate or carboxylate in an amount in the range of 1 to 30 wt% based on active ingredient as an engine oil for lubricating large low and medium speed engines in which the velocity at which interfacing surfaces of the engine move past each other when the engine is operating reaches at least 60 mm/s for improving the fuel economy of said engines, wherein said interfacing surfaces are selected from the group consisting of cylinder wall and piston and/or interfacing surfaces of bearings, wherein the improvement in the fuel economy is evidenced by the engine oil having a traction coefficient which is lower than the traction coefficient of engine oils which are not bimodal or which are based on Group I base oils or mixtures of Group I base oils and PIB, and wherein the traction coefficient is measured using an automated mini traction machine measurement instrument (MTM traction rig) at a temperature of 100°C, under a load of 1.0 GPa, a slide-to-roll ratio of 50% and a speed gradient of 0-3000 mm/s in 480 seconds. (a) a base oil comprising a bimodal blend of two different base oils, i. the first base oil being one or more oils selected from the group consisting of Group I base oils having a kinematic viscosity at 100°C of from 2 to less than 12 mm 2 /s and ii. a second base oil being one or more oils selected from Group IV base oils having a kinematic viscosity at 100°C of at least 38 mm 2 /s, the difference in kinematic viscosity between the first and second base oils being at least 30 mm 2 /s, and i. the first base oil being one or more oils selected from the group consisting of Group I base oils having a kinematic viscosity at 100°C of from 2 to less than 12 mm 2 /s and ii. a second base oil being one or more oils selected from Group IV base oils having a kinematic viscosity at 100°C of at least 38 mm 2 /s, the difference in kinematic viscosity between the first and second base oils being at least 30 mm 2 /s, and (b) one or more detergents selected from the group consisting of alkali and/or alkaline earth metal sulfonate, phenate, salicylate or carboxylate in an amount in the range of 1 to 30 wt% based on active ingredient as an engine oil for lubricating large low and medium speed engines in which the velocity at which interfacing surfaces of the engine move past each other when the engine is operating reaches at least 60 mm/s for improving the fuel economy of said engines, wherein said interfacing surfaces are selected from the group consisting of cylinder wall and piston and/or interfacing surfaces of bearings, wherein the improvement in the fuel economy is evidenced by the engine oil having a traction coefficient which is lower than the traction coefficient of engine oils which are not bimodal or which are based on Group I base oils or mixtures of Group I base oils and PIB, and wherein the traction coefficient is measured using an automated mini traction machine measurement instrument (MTM traction rig) at a temperature of 100°C, under a load of 1.0 GPa, a slide-to-roll ratio of 50% and a speed gradient of 0-3000 mm/s in 480 seconds. as an engine oil for lubricating large low and medium speed engines in which the velocity at which interfacing surfaces of the engine move past each other when the engine is operating reaches at least 60 mm/s for improving the fuel economy of said engines, wherein said interfacing surfaces are selected from the group consisting of cylinder wall and piston and/or interfacing surfaces of bearings, wherein the improvement in the fuel economy is evidenced by the engine oil having a traction coefficient which is lower than the traction coefficient of engine oils which are not bimodal or which are based on Group I base oils or mixtures of Group I base oils and PIB, and wherein the traction coefficient is measured using an automated mini traction machine measurement instrument (MTM traction rig) at a temperature of 100°C, under a load of 1.0 GPa, a slide-to-roll ratio of 50% and a speed gradient of 0-3000 mm/s in 480 seconds.