A method for improving fuel efficiency and energy efficiency, while maintaining or improving deposit control and cleanliness performance, in an engine lubricated with a lubricating oil by using as the lubricating oil a formulated oil. The formulated oil includes a base oil mixture. The base oil mixture includes a lubricating oil base stock as a major component, and at least one cobase stock, as a minor component. The at least one cobase stock is present in an amount sufficient to reduce kinematic viscosity (Kv100) of the base oil mixture as determined by ASTM D445, while maintaining or controlling cold cranking simulator viscosity (CCSV) of the lubricating oil as determined by ASTM D5293-15, such that the lubricating oil meets both kinematic viscosity (Kv100) and cold cranking simulator viscosity (CCSV) requirements for a SAE engine oil grade as determined by SAE J300 viscosity grade classification system. A lubricating oil having a composition including a lubricating oil base stock as a major component, and at least one cobase stock, as a minor component.