Multigrade Oil - To Use or Not to Use

Phil Ramsey

In my last column, multigrade oil was fairly prominent in my story. Let’s proceed from there, and From Under the Hood, discuss why we do, or don’t need multigrade oil in our engines.

The obvious reason to use multigrade oil is its superior pumpability in cold weather. In the southern part of the United States where I live, cold weather is described as anything that changes the physical properties of a swimming pool. Cold weather is something I wasn’t exposed to until I went to college in the North.

There I learned that antifreeze is not just coolant for your car. Where I’m from, we would use water in the engine instead, and save money on the antifreeze. After all, the temperature dropped below 26ºF (-3ºC) only once before I moved away from home, and then it was only for a few hours. However, multigrade was needed in states farther north, and years earlier along the Russian front during the winter of 1943.

Recently I saw a documentary on PBS about the Royal Air Force museum in England. A group was rebuilding a DB 601 engine from a German aircraft used in World War II. The Daimler Benz engine, now Daimler Chrysler, had an inverted-V 12-cylinder block and was supercharged, producing roughly 1,300 horsepower.

The Mechanic (Mechanic with a capital M for respect) restoring the engine couldn’t get over the tight tolerances of the war-manufactured engine. Because it was a liquid-cooled engine, it used the equivalent of what we know as 40W. It had no starter and was inertially cranked to start. This made it impossible to crank the nearly frozen oil in subzero temperatures along the Russian front.

Engines of up to 45 liters may be started with this Bosch model.
Daimler Benz DB601 60 degree V inline 2x6 turbo engine. From German aviation catalog circa 1939-40.

I remember the story of one Luftwaffe jagdstaffel (a division within the German Air Force) on the Russian front which flew the Me 109e and Me 109f planes with the DB 601 engines. The division lost more airplanes in January 1943 to fire than in combat.

The order to “fly operations at all costs” was given. Because the engine oils were frozen, a tarp was draped over the nose of the aircraft, and a low fire was built to thaw the engine and oil to a point where it could be cranked. The DB 601 was fuel-injected much like modern cars, and just as easy to start if you could get it turning.

Same as today’s fuel-injected engines, it burned 87(C3) or 92(B4) octane fuels. However there were leaks - due to a combination of wartime conditions with rough use, the tremendous expansion and contraction of fuel connections across the operating temperature of the engine, and winter temperatures. The fire within the tarp enclosure had at least 87 octane vapors floating nearby. No wonder the aircraft often caught fire.

Multigrade oil would have been helpful in those winter conditions. The cold pumpability of multigrades would have reduced the start-up procedures, but remember that it was -40ºF (-40ºC). Once the engine was up and running, what sort of oil was needed?

Cold weather protection, with slightly better performance on mileage, is the primary reason for using multigrade oils. Are there any drawbacks in its use? Let the owners manual for your vehicle help you decide. The manual for my 3.8L Buick says SAE 30 oil is preferred unless the cold start-up temperature is below 32ºF (0ºC) for several times between oil changes.

It further says that 15W-40 should be used in other situations, but recommends not using 10W-40 as it could void the warranty. SAE 30 is preferred because of its superior protection of engine bearings at operating temperatures. All choices involving viscosity are trade-offs, but if you can use monograde in your environment, or at least in the summer, do so.

It is interesting to me that owners of many later model cars will find 10W-30 suggested in their manuals. I’ll be honest: 10W scares me. The 5W-30 suggested by some also concerns me. In the southern part of the country, we don’t need 10W oils. Decreased friction of 10W and 5W oils does mean slightly better mileage.

But I wonder if it isn’t suggested for reasons of meeting corporate average fuel economy (CAFE) standards. This government regulation demands that vehicles meet increasing mileage levels. However, the question must be raised: Are we sacrificing engine life for slightly higher mileage to meet a government regulation? How much engine life is sacrificed? Is it cost-effective when compared to savings on gasoline costing $1.00 and $2.00 per gallon?

Considering the farther north you live, I know you have the need for multigrade oils. Even engine oil heaters are useful, but barely within my frame of reference. Imagine plugging your car into the electric grid even though you don’t live in California.

We will continue the multigrade discussion in upcoming issues. Use multigrade oil if you must, especially in colder climates. If you are basing your decision on the possibility of better mileage, here is a helpful hint: check your vehicle’s tire pressure. The next time you’re driving extended miles on the highway, inflate your tires to the recommended air pressure.

Carefully check your mileage over a 200-mile interval. Lower the air pressure five pounds and continue to check the mileage. My car improved three miles per gallon. Simply keeping your tires fully inflated will improve mileage more than using a multigrade oil.

What Is a Multigrade Oil?

A multigrade oil is an oil that meets the requirements of multiple grades — one “W” grade and one single grade, e.g., 5W-30 or 15W-40, etc.  This grading system has the advantage over the ISO system in that it defines the viscosity performance of the oil at both the low-temperature range and the high-temperature range.

multigrade oils are typically created by blending a low-viscosity base oil with VI improver additives (see the additives section of this book).  For example, when these polymer additives are blended in the correct proportion with an SAE 15W oil, the oil flows like an SAE 15W oil at low temperatures and like an SAE 40 oil at high temperatures.  The result is an SAE 15W-40 viscosity grade oil that will provide wide protection over an extended temperature range.  True multigrade oils will have a VI of 120 or greater and contain VI improver additives and/or synthetic base oils.

There are a couple of commercially available viscosity grades that appear to be multigrades (80W-90, 85W-140, 20W-20), but these oils have a VI of only about 100 and thus do not contain any VI improver additive or synthetic base oil.  It just happens that a straight mineral oil product can meet these viscosity grades.

Multigrade oils have several main advantages over single-grade oils.  These oils offer:

  1. One oil for year-round use.

  2. Improved low-temperature starting and less battery drain.

  3. Excellent high-temperature performance.

  4. Improved overall fuel economy by requiring less idling time, faster warm-up and providing high-speed temporary shear thinning (non-Newtonian behavior).

  5. Reduced wear by offering faster, full-pressure lubrication over a wider temperature range, i.e., an SAE 15W-40 oil provides full-flow lubrication in approximately 1 minute and 45 seconds at minus 25 degrees C.  This is a significant improvement over single-grade oils but is still inadequate for these temperatures.  A 5W-40 full synthetic diesel engine oil would be more appropriate.

Multigrade oils have a few disadvantages related to the VI improver additive:

  1. They cost more.

  2. They may permanently lose viscosity due to shearing (cutting up of) the large VI improver polymer, as illustrated on the right side of the following diagram.  In an extreme case, this could result in increased wear and failure.  This viscosity loss is detectable with oil analysis.

Overall, VI-improved multigrade oils provide far greater benefits than negative properties.

Consider This . . . By Drew D. Troyer

I thoroughly enjoyed reading this issue’s “From Under the Hood” column. As the article suggests, in the past decade the emphasis on lubricant viscosity selection has shifted from wear minimization to improved fuel economy and reduced emissions. However, lubricant and additive chemistry has improved over the past two decades. The machine remains well protected even though the oil’s viscosity is lower. Among the real benefits to using the lower viscosity oil are the following:

  1. According to the U.S. Department of Energy, a five percent reduction in fuel consumption reduces the demand for crude oil by approximately 100 million barrels per year in the U.S. alone.

  2. Emissions management is important to the environment, our health and quality of life. Small changes that are widely applied can add up to produce a large improvement.

  3. It is estimated that two-thirds of the frictional energy losses in an engine are hydrodynamic (fluid to fluid and surface to fluid). The increased heat produced by high viscosity lubes increases the rate at which seals, components and the lubricant degrade.

  4. Start-up wear is minimized with low viscosity oil (even in warmer climates) because the lube reaches its intended destination more rapidly.

Your SAE 10W oil is not your father’s 10W oil. Consider the following changes in lubricant formulation that have occurred during the past 20 years:

A. Modern PAO synthetic base oils and highly refined mineral oils have
naturally high viscosity indices (120 to 150 vs. 80 to 100 for older oils). In other words, a modern oil’s viscosity changes less for a given change in temperature, so it provides more protection at higher temperature, even though the oil is a lower viscosity grade.

B. Antiwear additives are better than ever. They are more stable and provide better boundary (surface to surface contact) protection than older technology additives.

C. Viscosity index (VI) improver technology has improved. The modern additives are much more shear-stable than older VI additives (the molecules resist being chopped up in the engine). Plus, because of the use of base oils with a higher natural VI, less of this additive is required.

Not only are lubricants better, but engines are more reliable too. Designs are better, machining tolerances are tighter and more consistent, fuel injection and electronic ignition have optimized combustion, materials technology is more precise - all of which add up to increased life and fewer problems during the typical service period.

Do you remember when 100,000 miles was good life out of a vehicle? Today, 150,000 miles and more is the norm, and we achieve this service life with a lot less maintenance. We may reminisce about how well-built cars were in the “good old days,” but if one looks objectively at reliability, the good old days are now (and getting better).

Another point is that most people don’t need more engine life from the typical consumer vehicle. While exceptions exist, consumer vehicles have a typical life of 12 years and 150,000 miles. Beyond this time frame, the following problems begin to arise:

A. Auxiliary system maintenance and nuisance failures become excessive.

B. The vehicle becomes an environmental nuisance due to leakage, reduced combustion efficiency and outdated (or out-of-service) emission control systems.

C. The vehicle’s styling becomes dated (some consumers are very style-conscious about their vehicles).

But the question about viscosity selection really comes down to dollars and cents. To justify using higher viscosity oil at the expense of fuel economy, the following statement must be true:

RC = Present value cost (today’s dollars) of an engine rebuild
FPV = Present value of fuel savings attributed to using low viscosity oil
PEF = Probability that the engine will fail before reaching the end of its required service life
PLE = Probability that engine failure could have been avoided by using high viscosity oil

During its life, the typical car burns between 6,000 and 7,500 gallons of fuel. Using the model above, let’s determine the rebuild cost required to justify a decision to use higher viscosity oil to avoid an engine failure.

FPV = $210, assuming a 3 percent reduction on 7,000 gallons purchased at one dollar per gallon (21.4 mpg)
PEF = 10 percent chance that the engine will fail before reaching 150,000 miles, our required life
PLE = 20 percent chance that a higher viscosity oil would have avoided the failure

RC = $210/(0.10 X 0.20)
= $210/0.02
= $10,500

So, the present value of an engine rebuild must exceed $10,500 (using my assumptions) to justify the use of higher viscosity oil, which is very high for a consumer vehicle. The calculation can be turned around to determine how much improvement in fuel economy is required to justify assumption of the risk of engine failure . . . all depending, of course, upon your assumptions about the risk of engine failure, and the likelihood that higher viscosity oil will reduce that risk.

For modern vehicles, the dollars and cents just don’t add up. While the life cycle fuel savings may seem small, they are large compared to the risk-adjusted cost of an engine rebuild. There are intangibles to consider as well, like the feeling you get from doing your part to reduce our impact on the environment and dependence upon fossil fuel.

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