is an engine oil specially formulated to increase fuel efficiency. A fuel-economy oil works by reducing the friction between moving engine parts that wastefully consumes fuel energy. There are two known means of accomplishing this goal: 1) by reducing the viscosity of the oil to decrease fluid friction and 2) by using friction-reducing additives in the oil to prevent metal-to-metal contact, or rubbing friction, between surface asperities.

Also see Energy Conserving/ Energy Conserving II.