I see statements and advertisements about the fuel efficiency benefits of certain engine oils. How does oil affect fuel efficiency, and what does the Fuel Efficiency Index tell me?

Fuel Efficiency Index

There are two ways in which the engine oil affects the fuel efficiency of the engine.

The first and most important way that engine oil affects fuel efficiency is through the viscosity of the engine oil. An oil’s viscosity is responsible for approximately 60% to 80% of the friction loss of the engine which affects the engine's fuel efficiency.

Viscosity is itself a form of friction — the viscous friction of oil molecules interacting with one another as the oil flows. This viscous friction is the most important property of oil in providing lubrication and preventing wear. That is, the oil’s viscosity slows the oil down and prevents it from easily escaping from those areas of the engine where, because it can’t escape fast enough, it becomes a thin platform between moving surfaces and protects them from damaging contact with one another. This response of the oil is called hydrodynamic lubrication. However, as mentioned under Operating Viscosity, even though higher operating viscosity is more protective, it also takes more energy to move the engine parts because of the viscous friction. So, higher viscosity is higher viscous friction and results in poorer fuel efficiency.

The second way that the oil can affect fuel efficiency is through certain additives put into the oil for this purpose when manufactured. These additives reduce engine friction in those areas operating under pressure and temperature conditions where hydrodynamic lubrication needs bolstering in attaining optimal lubrication.

The Fuel Efficiency Index is a measure of the effect of engine oil viscosity on fuel efficiency. A higher Index number indicates higher fuel efficiency.