Use of a lubricating oil having a kinematic viscosity at 100°C of 18 to 25 mm 2 /s and a base number of at least 5 mg KOH/g, and comprising: (a) a base oil comprising a bimodal blend of two different base oils, i. a first base oil being one or more oils selected from the group consisting of Group II base oils, Group III base oils and Group IV base oils having a kinematic viscosity at 100°C of from 2 to 16 mm 2 /s and ii. a second base oil being one or more oils selected from the group consisting of Group IV base oils having a kinematic viscosity at 100°C of 38 to less than 300 mm 2 /s, wherein the difference in kinematic viscosity between the first and second base oils in the bimodal blend is at least 30 mm 2 /s, and (b) a detergent selected from alkali and/or alkaline earth metal salicylates, phenates, carboxylates, mixtures of phenates and salicylates or mixtures of phenates and carboxylates at a total treat level in an amount of greater than 6 to 40 wt% (active ingredient), based on the total weight of the lubricating oil, as an engine oil for lubricating large low, medium and high 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 3 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 fuel economy is evidenced by traction coefficient of the engine oil employing the bimodal blend being lower than the traction coefficient of engine oils which are not bimodal blends or which are based on only Group I and/or Group II base oils or which contain no Group IV base oils having a KV at 100°C of at least 38 mm 2 /s, 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. a first base oil being one or more oils selected from the group consisting of Group II base oils, Group III base oils and Group IV base oils having a kinematic viscosity at 100°C of from 2 to 16 mm 2 /s and ii. a second base oil being one or more oils selected from the group consisting of Group IV base oils having a kinematic viscosity at 100°C of 38 to less than 300 mm 2 /s, wherein the difference in kinematic viscosity between the first and second base oils in the bimodal blend is at least 30 mm 2 /s, and i. a first base oil being one or more oils selected from the group consisting of Group II base oils, Group III base oils and Group IV base oils having a kinematic viscosity at 100°C of from 2 to 16 mm 2 /s and ii. a second base oil being one or more oils selected from the group consisting of Group IV base oils having a kinematic viscosity at 100°C of 38 to less than 300 mm 2 /s, wherein the difference in kinematic viscosity between the first and second base oils in the bimodal blend is at least 30 mm 2 /s, and (b) a detergent selected from alkali and/or alkaline earth metal salicylates, phenates, carboxylates, mixtures of phenates and salicylates or mixtures of phenates and carboxylates at a total treat level in an amount of greater than 6 to 40 wt% (active ingredient), based on the total weight of the lubricating oil, as an engine oil for lubricating large low, medium and high 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 3 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 fuel economy is evidenced by traction coefficient of the engine oil employing the bimodal blend being lower than the traction coefficient of engine oils which are not bimodal blends or which are based on only Group I and/or Group II base oils or which contain no Group IV base oils having a KV at 100°C of at least 38 mm 2 /s, 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.