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Recently I received an e-mail from Sue Ackley, past president of the Automotive Oil Change Association and owner of several Xpress Lube stores in the St. Louis area. She asked a very interesting question on the subject of changes in viscosity over the years.

Sue asked, Have you any information about the change in engine oils, how weights have changed over the years and how we now see more of the designer oils coming into play, like the SAE 0W-20, SAE 5W-40 Euro, SAE 5W-50, etc.? I have a customer who still uses SAE 10W-30 in EVERYTHING. I have tried to explain what has happened in the last 20 years, but what do I know?

I havent specifically written about the history of viscosity changes, although I did write a two-part article in February and March of 2010 on changes in the viscosity classification system and what impact that would have on engine oils. Here is my take on the question:

First, lets start with some definitions. Basically, viscosity is the resistance of any material to flow. The resistance is caused by the internal friction of the molecules moving against each other. Any material that has a plastic nature has a viscosity. For example, water is at the light end of the vis spectrum and such things as lard are at the heavy end. At 20 degrees C (68 F), lard is about 1 million times more viscous than water.

For a long time, engines were built using SAE 30 as the lubricant standard, and the absolute viscosity of SAE 30 at the operating temperature of the engine was used specifically for design purposes. That went on well into the 1980s. Thats when automobile and engine manufacturers finally recognized that lower viscosity oils have an inherent advantage over heavier oils in terms of friction reduction – lower viscosity means less internal friction in the oil. Lighter oils resulted in lower friction in the engine, which gave better fuel economy. Ever since, the bottom line in all of the reduced engine oil viscosity efforts is to improve fuel economy.

In the 1970s OEMs began to prefer multigrade engine oils, and SAE 10W-40 ruled as it gave excellent performance in a wide range of operating temperatures; it also had a fuel economy benefit versus SAE 30. The next step, SAE 10W-30, was a bit better in fuel economy than SAE 10W-40 and also provided good performance in a wide operating temperature range. This was also the era that saw friction modifiers introduced into oil formulations.

The 1980s brought the introduction of SAE 5W-30 which improved fuel economy even more, and with design changes to the engine (such as roller followers instead of flat tappet valve lifters) offered good protection for the hardware. These oils gave even better low-temperature performance. SAE 5W-30 was king of the hill for the next 20 years or so. In addition, there were improvements in friction modifiers as well as the introduction of ASTM engine sequence tests to measure fuel economy. Adoption of 5W-30 was slow to take off, but today it accounts for an estimated 47 percent of the motor oil viscosity demand for all U.S. cars and light trucks.

Starting about 2000, newer and even lighter viscosity grades such as SAE 5W-20, 0W-20 and SAE 0W-30 began to show up in the general marketplace. These oils also showed an improvement in fuel economy over earlier oils and, again due to improving engine designs, gave good protection.

So from its once-dominant spot on top of the viscosity pile, SAE 10W-40 today is only 6 percent of the U.S. passenger car engine oil market, while 5W-30 is the market heavyweight with a 47 percent share, according to Infineum.

You should be able to see a trend here. The W side of the SAE grade has the greatest impact on fuel economy, while the second viscosity number is, well, secondary. In addition the friction modifiers present also add to the oils fuel economy benefits.

This is a good time to ask how fuel economy became so important for engine oils, to where it seemingly outweighs other aspects like wear protection, oxidation stability, cold cranking and evaporation. And here is where government and big money raise their heads.

In the 1970s, legislation was passed mandating that average fuel economy for each passenger car manufacturer should be a minimum 27.5 miles per gallon, to be reached by the 1985 model year. This is the so-called CAFE, or corporate average fuel economy requirement. It simply means that the average fuel economy of all engine-and-transmission combinations for vehicles sold in the United States by any OEM must meet or exceed 27.5 mpg. At the time this legislation was passed in 1977, the average fuel economy of automobiles in the United States was around 15 mpg.

The OEMs knew that their best-selling and highest-margin vehicles such as SUVs, vans, full-size automobiles and pickups, couldnt realistically meet that target and deadline. So they built small vehicles with very high fuel economy to offset the fuel guzzlers; averaged together, the fleet would hit the target. They also knew that the federal test methods required to measure CAFE had a degree of variability that might peg their fleets at 27.2 or 27.3 mpg, not the mandated 27.5, so they needed some margin of safety.

According to the law, the financial penalties for not meeting the CAFE targets were pretty severe: $10 per 0.1 mpg below 27.5, per unit sold. If an OEM built 1 million vehicles and its CAFE clocked in just 0.2 mpg below the minimum, that OEM faced a punishing $20 million fine. Lighter viscosity engine oils alone could give 2 to 3 percent better fuel economy – or 0.55 to 0.82 mpg improvement.

However, the law also said that, for the OEMs to claim the benefits of fuel-economy engine oils, the oils needed to be widely available and cost competitive with current oils. That meant no exotics.

So we saw an approach to addressing CAFE which involved both reduced viscosity for engine oils as well as a careful balancing of vehicle product mix. Some of the small vehicles from that era which would give higher mpg were the Ford Fiesta, Chevy Citation and Plymouth Reliant. Those were coupled with the larger, less fuel efficient products to achieve the 27.5 mpg target.

A similar situation exists today, with new legislation which will require OEMs post CAFE numbers of 35.5 mpg beginning in 2016, not even four years away. That brings immediate pressure to develop more fuel-miserly vehicles and to reduce oil viscosities even more, as the lubricants industry starts to wrestle with the next engine oil upgrade.

Hybrids and all-electrics are one of the ways to gain a fuel economy edge while still selling the vehicles that owners want. Heres an example: Suppose you are an OEM producing an all-electric vehicle that delivers a reported 100 mpg. (I know its not exactly accurate to define mpg values for an electric vehicle but the government has worked out a method of making that sort of calculation.) If your target CAFE is 35.5 mpg, one electric powered vehicle can offset four conventionally powered vehicles that average 20 mpg. Youll average 36 mpg for all five models, and with that mix, you can nail the CAFE requirement and still make the vehicles most desired by customers.

The answer Sue needs to give her customer is that he should check the owners manual for each vehicle he owns and use the recommended viscosity grade. Modern engines are designed around their recommended viscosity, and will get the very best mpg as well as long and satisfactory engine life. There should not be any problem with wear protection of the engine, even with SAE 0W-20 if that is the recommended oil grade.

The oil industry has been on a downward slide with respect to viscosity for a long time now, decades in fact. Those of you who read this column regularly know that I am concerned with viscosities getting too low and risking engine oils that cannot provide adequate protection. Im also concerned that the continued drive to lowered viscosities will cause a major upheaval in refining processes.

On more than one occasion Ive written about the overall average (or pool) viscosity of base oils, and how they have continued to drop. In order to supply lower viscosity base oils, there will continue to be more and more hydrocracking. That gets us to lower viscosity base oils but sacrifices the higher-viscosity materials needed for other applications such as gear oils, stationary engines, marine and railroad engines.

Im also concerned that single-grade oils will suffer as well, which could be a serious problem for certain applications such as auxiliary engines in nuclear power plants.

For Sue Ackleys customer, adopting the OEMs recommended viscosity for his vehicle(s) is a good thing. For other applications, viscosity continues to be the single most important oil property, and changes need to be made cautiously and with great care.

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