The other day I was looking through a trade magazine and came across a brief article about engine oils and fuel economy. A reader had switched from using a mineral oil based SAE 10W-40 to a synthetic SAE 5W-30, reasoning he would get longer drain intervals. However, he was curious about what fuel economy benefits he might achieve through the use of a lower viscosity engine oil.

The magazines response focused on the fuel economy benefit of lowering viscosity. As the article pointed out, lower viscosity means less internal friction in the fluid, which should improve fuel economy. Furthermore, the response said, lowering the oils viscosity by one grade would give about a 1 percent fuel economy improvement.

It went on to explain that a 1 percent improvement for a vehicle getting 40 miles per gallon would be 0.4 miles per gallon, so a 15-gallon fuel tank would have a net gain in the range of six miles per tankful of gasoline. Calculating from that basis and assuming gasoline costs $2.50 a gallon, the reader would save about $6 in fuel costs over 10,000 miles (the suggested drain interval for synthetic SAE 5W-30).

The problem I have with this analysis of fuel economy benefits, like others Ive read, is that it doesnt jibe with my understanding of fuel economy. While its clear that lower viscosity generally improves fuel economy, Im not comfortable making the correlation between viscosity grades.

First, lets get a dose of reality here. Theres no telling how much youll actually save, since driving habits – lead foot or light touch – driving cycle (traffic vs. open road), and vehicle age and condition all impact fuel economy. Open the sunroof or load up the trunk or fail to maintain the right tire pressure, and you could have a major impact on fuel economy.

The article also drew attention to concerns that lower viscosity oils may impact engine durability. There may be some validity to this, but not as much as many of you might think. Along with lighter viscosity have come advances in additive chemistry and engine manufacturing, such as improved tolerances in engines, that boost efficiency. In fact, heavier oils may be less than optimum for long engine life.

I can testify to that point by noting that one of our family vehicles was a 1974 Dodge station wagon. It was a nice vehicle that we bought new, and used to move from California to St. Louis, Missouri, and back. It withstood five winters, some of the worst in a long time. By the time it had about 60,000 miles on the odometer, it was really having trouble engine-wise. By contrast, my 2001 GMC pickup had 98,000 miles on it when I sold it last year, and its new owner says it is still going great. Id say that speaks to improvements in manufacturing.

Since the mid-1970s, a great deal of progress has been made in engine design and operation. The advent of the on-board computer opened a new world of fuel combustion control. Timing could be managed with great precision, and fuel injection, which had been very limited in scope, now became the standard, replacing carburetors. You now have the right amount of fuel going to the right place at the right time to not only gain fuel economy but also to reduce emissions, which is the twin sister of fuel economy.

Also, remember that fuel economy is primarily a gross effect over all vehicles and types. Federal regulations are built on the concept that automakers need to meet Corporate Average Fuel Economy targets across their entire fleet. Each 1 percent reduction in fuel consumption over 50 percent of vehicles on the road represents an enormous gain in fuel economy for the United States.

In 2012, U.S. light duty vehicles (essentially passenger cars and light trucks) consumed 15.3 quads of energy. Do you know what a quad is in this context? I looked it up, and its a quadrillion BTU – a 1 followed by 15 zeros! Regular gasoline with 10 percent ethanol typically has about 120,000 BTU per gallon. That, my friends, means 1.275 X 1011 (127.5 billion) gallons of gasoline were consumed in the U.S. in 2012. And 1 percent of that is 1.27 billion gallons. Wow!

With the auto industry facing a targeted 20 percent increase in CAFE in 2017, and an additional 54 percent increase in 2025, you can see the extreme importance of fuel economy. It is only equaled by targeted reductions in CO2 emissions, which are similar in magnitude to the fuel economy improvements.

Getting back to the relative merits of an SAE 5W-30 synthetic vs. a conventional SAE 10W-40, the greatest impact on fuel economy really is the oils high temperature, high shear viscosity. Reducing HTHS viscosity is deemed so important that we now have SAE 0W-30, SAE 0W-20 and even SAE 0W-16 oils in the marketplace. These grades will further improve the CAFE outcomes for gasoline-fueled engines.

Long ago, most consumers learned that multigrade is good. Some (maybe most) read their owners manuals and find the viscosity grades recommended for their car or light truck are multigrades. However, there are a few who still cling to the idea that monogrades are the way to go. Some worry that multigrade engine oils will be too light for their vehicle, especially if they live in a warm environment.

I remember a man that I used to van-pool with to work in Houston. He had an old Fiat that was an oil burner of the first magnitude. In fact, we could see the smoke before we saw the car as he drove to our rendezvous site (a bit of hyperbole here). About once a month, he would go to the local grocery store and buy a case of the lowest-priced monograde SAE 30 motor oil. He was quite pleased that he could put a quart in every few days without spending much money. Besides, he always said, monograde was best. Turned out that he was buying API SA oil, and it wasnt doing the engine a lot of good. At any rate, it wasnt long before he came to meet the van in a new car. But I digress.

One of the worries that occupies most mechanics regarding viscosity is that if the viscosity is too low, there will be increased wear. They reason that if the oil is thicker it will protect the moving parts better. In point of fact, the lower the viscosity is at startup, the quicker it gets to the critical engine parts, such as the valve train, and begins to lubricate. The result is less wear and longer engine life.

Of course using a multigrade engine oil means using an oil with a polymer to thicken what is essentially a lightweight base oil. SAE 30 is made from base oil with a viscosity of around 10 centiStokes at 100 degrees Celsius. By contrast, SAE 5W-30 is made from base oil with a viscosity of around 4 cSt at 100 C.

Shouldnt the heavier oil protect better? As I just pointed out, lighter oil reaches the surfaces it needs to protect quicker and at lower temperatures (startup). When the oil reaches the point of high shear, a monograde will be viscous and create drag, lowering fuel economy without any discernable improvement in protection.

At operating temperatures, multigrade oil behaves like a heavy oil – except in the high shear rate areas of the engine such as bearings, cylinder liners and valve train. In these high shear zones, multigrade engine oils will assume an alignment with the direction of the motion, especially the polymeric thickener, which will temporarily reduce the viscosity. The result is easier movement of parts, cooler operation and improved fuel economy.

So if youre an oil marketer, what do you tell a customer who wants to put in one kind of oil, when you know it would be better to use something else? Heres a checklist to help them make the right decision. Remember, you want their return business.

What does her owners manual say? Thats the first place to go for oil recommendations.

What kind of driving does he do? Is it as a commuter, or is he pulling a trailer?

What is his price point? Is he willing to pay for a synthetic, or is conventional oil good enough?

How old is the vehicle? Older vehicles may need a high-mileage oil which offers some additional seal swell performance.

It seems like choosing the right oil is an objective decision, but to many, it is based more on personal history – your fathers fathers oil, as some would say – or name recognition or brand loyalty or something else. Being a real expert is your most valuable selling tool.

Industry consultant Steve Swedberg has over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at steveswedberg@cox.net.