Lubricants with improved low-temperature fuel economy

Abstract

Lubricating oil compositions, formulated to a preselected viscosity grade that demonstrate stay-in-grade capability in a diesel injector shear stability test, are provided. The compositions comprise a major amount of a base oil of lubricating viscosity and a minor amount of (i) a viscosity modifier (VM) or mixtures thereof having a low shear stability index (SSI) and (ii) a VM or mixtures thereof having an SSI greater than that of the VM from (i).

What is claimed is: 
     
         1 . A lubricating oil composition formulated to a preselected SAE engine oil viscosity, said composition comprising:
 (a) a major amount of a base oil of lubricating viscosity; and   (b) a minor amount of
 (i) a viscosity modifier (VM) or mixtures thereof having a low shear stability index (SSI), and 
 (ii) a VM or mixtures thereof having an SSI greater than the VM from (i); 
   (c) wherein the sheared kinematic viscosity at 100° C. of the composition after 90 cycles in the diesel injector shear stability test (ASTM D7109) is equal to or greater than the minimum viscosity for the preselected grade before shearing; and   (d) wherein the sheared viscosity (c) is less than 25% lower relative to the unsheared composition viscosity.   
     
     
         2 . The composition of  claim 1  wherein the VM's of (i) therein have an SSI in the range of 4 to 12. 
     
     
         3 . The composition of  claim 1  wherein the VMs of (ii) have an SSI in the range of 8 to 65 or higher. 
     
     
         4 . The composition of  claim 2  wherein the VM weight ratio of VM(ii) to VM(i) is in the range of 0.01 to 1.5. 
     
     
         5 . The composition of  claim 3  wherein the VM weight ratio of VM(ii) to VM(i) is in the range of 0.05 to 1. 
     
     
         6 . The composition of  claim 4  wherein the total amount of VM(i) and VM(ii) comprises from 0.1 wt % to 2.5 wt % of the total weight of the composition. 
     
     
         7 . The composition of  claim 5  wherein the total amount of VM(i) and VM(ii) comprises from 0.1 wt % to 2.5 wt % of the total weight of the composition. 
     
     
         8 . The composition of  claim 1  wherein the VMs are selected from vinylaromatic-diolefin copolymers. 
     
     
         9 . The composition of  claim 1  wherein the VMs are selected from olefin copolymers and vinylaromatic-diolefin copolymers. 
     
     
         10 . The composition of  claim 1  wherein the base oil comprises one or more base oils selected from Group III and Group IV oils and mixtures thereof. 
     
     
         11 . The composition of  claim 1  comprising one or more lubricant additives, selected from oxidation inhibitors, dispersants, detergents, corrosion inhibitors, metal deactivators, antiwear additives, extreme pressure additives, pour point depressants, seal compatibility agents, friction modifiers, defoamants and dyes. 
     
     
         12 . In the method of lubricating an internal combustion engine by supplying a lubricating oil composition to the engine, the improvement comprising supplying an SAE multigraded lubricating oil composition to the engine, said oil composition comprising any of the compositions of  claims 1  to  11 , thereby enhancing the fuel efficiency of the engine while providing stay-in-grade viscosity retention for stable engine performance.