It depends on how your engine is built and how it reacts to the specific viscosity of oil you use. That’s pretty nebulous, so here are some examples.
Why does honey flow more slowly than water? Because of the difference in its viscosity. Viscosity is the measure of a fluid’s resistance to flow. Water, for example, flows much faster than honey or maple syrup (if you prefer that in your morning coffee) when poured from a container. That’s because water has a lower viscosity.
Although gas is relatively cheap these days, increasing fuel economy is still a top priority for most automakers, due in large part to more strict corporate average fuel economy (CAFE) requirements on the horizon. One strategy automakers are using is widespread use of synthetic lubricants. How do synthetics increase fuel economy compared to conventional lubricants?