Fuel economy benefits derived from improved engine crankcase lubricants have been well documented in the last 10 years. These benefits can be gained not only by a reduction in oil viscosity but also by addition of special friction modifier additives. These have been shown to operate by reducing the friction within the boundary regimes of the engine.Work has been carried out which indicates that a secondary but possibly more significant contribution can be made by friction modifiers where the hydrodynamic regime is extended to lower viscosities. As a result it may be possible to formulate finished lubricants with lowered viscosities while remaining free from wear debits. Such optimised lubricants will result in furtner fuel economy improvements.