Do thinner motor oils offer better fuel economy and less wear?

Although synthetic motor oils are increasingly being used, mineral lubricants are probably still in the majority which often leads to a discussion around the best engine oil viscosity for the application and whether thinner motor oils hold any meaningful benefits for the motorist.

Conventional wisdom states that engine oils that increase fuel
economy allow less friction and prolong engine life. Conventional thinking also equates these properties with thinner motor oils.

Thinner motor oils could improve fuel economy, but does anyone really care?

First, we should face the fact that the consumer does not typically care about fuel economy except during difficult economic times.

This can be observed by how most vehicles are driven. Anyone accelerating slowly or driving at the speed limit to conserve energy is a danger to himself and other drivers who’s driving style is usually more focused on saving time than fuel.

Auto manufacturers, on the other hand, are concerned about fuel economy. Either as a result of negative sales publicity or the additional taxes applied for vehicles that consume more fuel.

Thinner motor oils seem to be the answer to fuel consumption.

Thinner motor oils are being used these days for three reasons: They save fuel in test engines (This is important to note), the viscosity rules have changed, and manufacturers are recommending thinner grades.

The Sequence VI-B is the test used to evaluate fuel economy for the GF-3 specification. The VI-B test engine is fitted with a roller cam where the old Sequence VI test used a slider cam. The old Sequence VI test responded well to friction modifiers, but the Sequence VI-B responds to thinner motor oils.

The test oil’s fuel efficiency is compared to the fuel efficiency of a reference oil in the Sequence VI-B test. To pass, the test oil must improve fuel economy one to two percent, depending on viscosity grade. SAE 5W-20 must produce higher relative fuel efficiency than SAE 5W-30.

It is interesting to note that the reference oil is fully PAO synthetic SAE 5W-30. To qualify for the GF-3 Starburst, ordinary mineral oils had to beat the fuel economy of the full synthetic reference oil. (It seems there is more to fuel economy than a magic base oil.)

Another factor in fuel economy is temporary polymer shear. These polymers are additives known as viscosity index improvers (or modifiers). Polymers are plastics dissolved in oil to provide multiviscosity characteristics. Just as some plastics are tougher, more brittle or more heat-resistant than others, different polymers have different characteristics.

Polymers are huge molecules with many branches. As they are heated, they uncoil and spread out. The branches entangle with those of other polymer molecules and trap and control many tiny oil molecules. Therefore, a relatively small amount of polymer can have a huge effect on oil viscosity.

As oil is forced between a bearing and journal, many polymers have a tendency to align with each other, somewhat like nesting spoons. When this happens, viscosity drops. Then when the oil progresses through the bearing, the polymer molecules entangle again and viscosity returns to normal. This phenomenon is referred to as temporary shear.

Because the Sequence VI-B test responds to reductions in viscosity, oil formulators rely on polymer shear to pass the test. A shear stable polymer makes passing the GF-3 fuel economy test much more challenging.

New rules defining the cold-flow requirements of SAE viscosity grades (SAE J300) were introduced in June 2001, as the auto manufacturers were afraid that modern injection systems might allow the engine to start at temperatures lower than the oil could flow into the oil pump. Consequently, the new rules saw a thinning effect on oil.

The auto manufacturers now recommend thinner motor oils for their vehicles than in the past. Years ago, SAE 10W-40 was the most commonly recommended viscosity grade, later migrating to SAE 10W-30. SAE 5W-30 is most popular now, but several manufacturers are now recommending SAE 5W-20 or even SAE 0W-20. 

Because of the change in cold-flow requirements and the fuel economy test pushing formulators toward the bottom of the viscosity grade, today’s SAE 10W-30 oils are more like yesterday’s (GF-1 spec) SAE 5W-30 oils. On top of that, there is a trend toward auto manufacturers recommending thinner grades.

Thinner motor oils cause wear.

Thinner motor oils have less drag, and therefore less friction and wear. Right?

Perhaps in the test engine or engines that experience normal operation. But somewhat thicker oils probably offer more protection for more severe operations such as driving through mountains, pulling a boat, dusty conditions, short trips, high rpm, overloading, overheating and overcooling.

Any abrasive particles equal to or larger than the oil film thickness will cause wear. Filters are necessary to keep contaminants small. The other side of the equation is oil film thickness. Thicker oil films can accommodate larger contaminants.

Temperature has a major influence on viscosity and film thickness. As a point of reference, one SAE grade increase in viscosity is necessary to overcome the influence of a 20°F increase in engine temperature. At a given reference point, there is approximately a 20°F. difference between viscosity grades SAE 30, 40 and 50. SAE 20 is somewhat closer to 30 than the other jumps, because SAE 30 must be 30°F higher than SAE 20 to be roughly the equivalent viscosity.

In other words, an SAE 20 at 190°F is about the same kinematic viscosity as an SAE 30 at 220°F, which is about the same viscosity as an SAE 40 at 240°F. This approximation works well in the 190°F to 260°F temperature range.

If an SAE 50 oil at 260°F is as thin as an SAE 20 oil at 190°F, imagine how thin the oil film becomes when you are using an SAE 5W-20 and your engine overheats. When an engine overheats, the oil film becomes dangerously thin and can rupture.

Short-term Thinking on using thinner motor oils.

As wear increases, the efficiency of an engine declines. Valve train wear slightly changes valve timing and movement. Ring and liner wear affect compression. The wear hurts fuel efficiency and power output by an imperceptible amount at first, but then the difference in fuel economy between an SAE 10W-30 and SAE 5W-20 or even is hardly noticeable. Efficiency continues to decline as wear progresses. Perhaps optimizing wear protection is the way to reduce fuel consumption over the life of the engine.

Certainly engines that have experienced significant ring and liner wear benefit from thicker oils. Thicker oil use results in compression increases, performance improvements and reduced oil consumption.

High-mileage oils are a relatively new category of passenger car motor oils. These products typically contain more detergent/ dispersant and antiwear additives than new car oils. They typically contain a seal swell agent and are available in thicker viscosity grades than most new cars recommend. “High mileage” seems to be defined by “as soon as your car is out of warranty.”

Should you be using thinner motor oil?

Although thinner motor oils with less antiwear additive outperform more robust products in the 96-hour fuel economy test, it is not clear that such products save fuel over the useful life of the engine.

Oils recommended by the auto manufacturers seem to compromise protection from wear under severe conditions to gain fuel economy and catalyst durability. It is important to recognize that to use a product that offers more protection from wear will most likely compromise your warranty. Thicker oils also compromise cold temperature flow, which may be of concern depending upon climate and season.

The best protection against wear is probably a product that is a little thicker (such as SAE 10W-30 or 15W-40) and has more antiwear additives than the oils that support the warranty.

The best synthetic motor oil for your vehicle depends on your driving habits, the age of your engine and the climate you drive in, but it is not necessarily the type of oil specified in the owner’s manual or stamped on the dipstick.

This article is focused on mineral oils where compromise is often the name of the game. However quality synthetic oils, such as those produced by Habot Synthetic lubricants are engineered to deliver both performance and durability. This means there’s no need to compromise wear, such as would be the case using thinner mineral motor oils, to achieve optimum fuel consumption and performance.