5W-20 oil is a thinner oil with lighter viscosity that creates less drag on the crankshaft, pistons and valvetrain. Additionally, the oil pump can pump thinner oil more easily, improving oil circulation. Any increase in fuel economy may not be noticed by the average motorist. Machined internal engine parts are more precise than the parts of 20 years ago. This means that clearances between moving parts are smaller and more exact. Thinner oil such as 5W-20 can flow more freely through the engine while still filling the spaces. Thicker oil is harder to push through the spaces between the parts. This causes the oil pump to work harder, which in turn increases oil pressure while simultaneously decreasing oil volume. A lack of oil volume results in a decrease of lubrication and cooling, which may decrease engine part life.
The lighter viscosity of 5W-20 oil flows faster at start-up compared to higher viscosity oils, which helps reduce engine wear in critical areas by lubricating parts faster. Valvetrain components at the top of the engine require immediate lubrication at start-up.
Oil additives are not recommended as noted in the owners manual. The American Petroleum Institute (API) certifies that oils such as Motorcraft 5W-20 already contain the necessary additives for friction, detergent, etc... The addition of additives may interfere and react with the additives already present in the certified oil.