In US and Japan, when you buy motor oil, chances are that it will have the ILSAC starburst symbol on it, which implies among other things that you are getting the lowest viscosity allowable for the SAE viscosity grade you are purchasing. This is in sharp contrast to oils sold in Europe, where most oils are in the thick range. This is because there is a widespread belief that thinner the oil you use, you would get better fuel economy. This has been driven mostly by US and especially Japanese OEMs' quest for squeezing every possible fractional gain in fuel economy. The simplistic motivation is that for hydrodynamically lubricated systems, internal frictional energy loss inside the oil decreases with decreasing viscosity.
However, in reality, this is not as simple. First, only the bearings and parts of the cylinders and rings are hydrodynamically lubricated. Second, the driving conditions can put them outside the hydrodynamic region. Third, as oils are getting thinner and thinner, they are being put outside the hydrodynamic-lubrication region and into the mixed-lubrication region.
The real evidence comes from the official whitepaper of ILSAC on their fuel-economy test -- Sequence VID (link).
They studied various viscosities with and without different kinds of friction modifiers and for different operating conditions (stages), the latter of which are summarized in the first table. The results are summarized in the second table. If Oil B - Oil A is positive, B has better fuel economy than A, and if it's negative, B has worse fuel economy than A and so on. Yellow highlighting means statistically significant result.
The undisputed conclusion is that both moly (inorganic) and organic (nonmetal) friction modifiers (FM) increase the fuel economy substantially. We also know that these compounds decrease wear. So, they are win - win additives in engine oils.
What is not clear is what the viscosity does. In many cases, thinner oil actually results in worse fuel economy (with the same FM additives) and thicker oil results in better fuel economy. It also widely depends on operating conditions (stages).
In many other cases, the results are not statistically meaningful.
So, what's going on? The answer lies in the fundamental curve of lubrication -- the Stribeck curve, which plots the friction as a function of viscosity , RPM (v), and load pressure (P). There are three regions: boundary lubrication (metal-to-metal contact, highest friction, highest wear), mixed lubrication (some metal-to-metal contact with widely wearing friction), and hydrodynamic lubrication (no metal-to-metal contact at all, no wear at all, but friction increases with viscosity). What's happening is that OEMs have been trying to reduce the viscosity to the area where mixed- and hydrodynamic-lubrication regions meet, which has the lowest friction. However, since different parts of the engine has different load pressures (P), they can't all be in the same region. Moreover, the load pressures vary widely with operating conditions such as RPM and power output. Therefore, when you lower the viscosity, you also run the risk of increasing the friction and wear by going toward further leftward on the Stribeck curve, into the left part of the mixed lubrication and into the boundary lubrication, with increasing metal-to-metal contact.
Moral of the story: do not outright assume that the OEM viscosity recommendation will either result in better fuel economy or less wear. You may get neither benefit but worse fuel economy and increase wear. Your driving conditions should also be a major factor in choosing the right viscosity for your engine. (High speeds, high loads [such as uphill driving], towing, etc. require thicker oil.) Also, base-oil quality and friction modifiers affect the fuel economy greatly, with quality synthetic oils (PAO, GTL, Group III+, etc.) loaded with quality inorganic (trinuclear moly etc.) and state-of-the-art organic friction modifiers giving the best fuel economy.
However, in reality, this is not as simple. First, only the bearings and parts of the cylinders and rings are hydrodynamically lubricated. Second, the driving conditions can put them outside the hydrodynamic region. Third, as oils are getting thinner and thinner, they are being put outside the hydrodynamic-lubrication region and into the mixed-lubrication region.
The real evidence comes from the official whitepaper of ILSAC on their fuel-economy test -- Sequence VID (link).
They studied various viscosities with and without different kinds of friction modifiers and for different operating conditions (stages), the latter of which are summarized in the first table. The results are summarized in the second table. If Oil B - Oil A is positive, B has better fuel economy than A, and if it's negative, B has worse fuel economy than A and so on. Yellow highlighting means statistically significant result.
The undisputed conclusion is that both moly (inorganic) and organic (nonmetal) friction modifiers (FM) increase the fuel economy substantially. We also know that these compounds decrease wear. So, they are win - win additives in engine oils.
What is not clear is what the viscosity does. In many cases, thinner oil actually results in worse fuel economy (with the same FM additives) and thicker oil results in better fuel economy. It also widely depends on operating conditions (stages).
In many other cases, the results are not statistically meaningful.
So, what's going on? The answer lies in the fundamental curve of lubrication -- the Stribeck curve, which plots the friction as a function of viscosity , RPM (v), and load pressure (P). There are three regions: boundary lubrication (metal-to-metal contact, highest friction, highest wear), mixed lubrication (some metal-to-metal contact with widely wearing friction), and hydrodynamic lubrication (no metal-to-metal contact at all, no wear at all, but friction increases with viscosity). What's happening is that OEMs have been trying to reduce the viscosity to the area where mixed- and hydrodynamic-lubrication regions meet, which has the lowest friction. However, since different parts of the engine has different load pressures (P), they can't all be in the same region. Moreover, the load pressures vary widely with operating conditions such as RPM and power output. Therefore, when you lower the viscosity, you also run the risk of increasing the friction and wear by going toward further leftward on the Stribeck curve, into the left part of the mixed lubrication and into the boundary lubrication, with increasing metal-to-metal contact.
Moral of the story: do not outright assume that the OEM viscosity recommendation will either result in better fuel economy or less wear. You may get neither benefit but worse fuel economy and increase wear. Your driving conditions should also be a major factor in choosing the right viscosity for your engine. (High speeds, high loads [such as uphill driving], towing, etc. require thicker oil.) Also, base-oil quality and friction modifiers affect the fuel economy greatly, with quality synthetic oils (PAO, GTL, Group III+, etc.) loaded with quality inorganic (trinuclear moly etc.) and state-of-the-art organic friction modifiers giving the best fuel economy.