Motorcycle Oil vs. Automotive Oils

Surprising New Evidence on the Viscosity-Retention Question

So The Story Goes ...

Many motorcyclists have long doubted the need to pay the inflated prices asked for most motorcycle-specific engine oils. An even larger number of us have harbored at least some degree of skepticism about the claims made for motorcycle oils, but have been reluctant to turn away from them, for fear of damaging our precious machines if the claims should happen to be true. Most of this fear comes from very successful marketing campaigns mounted by the manufacturers and distributors of motorcycle-specific lubricants.

For example, a monthly trade publication for motorcycle dealers recently published an article suggesting, "negative selling techniques" to "educate customers" against purchasing automotive oil for their bikes. The example in the article begins with the benevolent dealer looking the poor, dumb customer in the eye and asking, in an incredulous voice, "You're not really using that in your motorcycle, are you?"

The idea, of course, is not so much to educate as to frighten the customer into paying for the more expensive motorcycle oil that only guess-who sells. Such techniques have played on our fears with great effect, to the point where high-priced, motorcycle-specific lubricants have become staple profit producing items in the majority of motorcycle dealership parts departments throughout the country.

The campaigns promoting motorcycle-specific oils have successfully indoctrinated an entire Generation of motorcycle riders and mechanics. The doctrine is now so ingrained in the industry that questioning its veracity instantly marks you as an ill-educated outsider. Even MCN has fallen victim to the hype, espousing the superiority of such products in these very pages. Our own technical experts from the American Motorcycle Institute have repeatedly advised our readers against the dangers of straying from the straight and narrow path.

What we, as well as the AMI, your local mechanic and all the other motorcycling publications have been doing is simply repeating what we have been carefully taught to believe over the years. The only problem with this approach is that our only source of information has been the people who stand to profit from our faith in the superiority of motorcycle-specific oils.

Stretching the Truth - Just a Bit

Motorcycle oil producers make a multitude of claims for their products, some of which are extremely difficult to substantiate, and others which are simply outdated and no longer applicable. This is not to say that all claims made for the superiority of motorcycle oils are necessarily false, only that the actual differences between them and their automotive counterparts may be considerably less than we have been lead to believe. For example:

Claim - Since the introduction of catalytic converters in automobiles, the best anti-wear agents have been limited by law to the amount that can be used in automotive oils. but are present in greater concentration in motorcycle oils.

Fact - Phosphorous deteriorates the catalyst in converters and is therefore restricted to a very small percentage in automotive oils. Phosphorous is also an essential element in one of the best anti-wear agents, ZDDP (zinc dialkyldithiophosphate), which is a primary component of such over-the-counter engine additives as STP Engine Treatment.

While it is true that slightly increased concentrations of ZDDP are found in some motorcycle oils (such as Spectro products), it is also true that these concentrations still fall under the governmental limits, otherwise these oils could not be used in the new converter-equipped motorcycles from BMW and Yamaha. Also, it should be noted that ZDDP is a "last line of defense"-type additive, generally only coming into play under extremely severe conditions where actual metal-to-metal contact occurs within an engine, something that should never happen under normal operating conditions.

Claim - Motorcycle engines run hotter and rev higher than automobile engines, therefore requiring oils with more expensive, shear-stable polymers and additives than automotive oils.

Fact - This is one of those statements that was much more true in the 1970s than in the 1990s. The big, slow-revving Detroit automobile engines of the past have mostly been replaced with smaller, higher-revving four-cylinder and six-cylinder engines that have much more in common with their counterparts running on two wheels. Keeping pace with the development of the small, high-revving, automobile engine, automotive oils have improved considerably, to the point where the newer, SG-rated automotive oils are nearly identical to motorcycle oils.

In most cases where motorcycle oil producers show comparisons between their products and automotive oils, you will find them using SE- or SF-rated oils as the "automotive standard." These are oils that were designed and rated for the cars of 10 to 20 years ago. We have yet to see a motorcycle oil compared in testing to the 1990's standard, SG-rated premium automotive oils.

The Viscosity-Retention Claim

By far the loudest and most-believed claim made for motorcycle oils is that they retain their viscosity longer than automotive oils when used in a motorcycle. The standard claim made in most advertising is that motorcycle-specific oils contain large amounts of expensive, shear-stable polymers that better resist the punishment put on the oil by the motorcycle's transmission, thus retaining their viscosity longer and better than automotive oils would under the same conditions.

This quote comes directly from the back of a bottle of Spectro 4 motorcycle oil, and is similar to the advertising line used by nearly all motorcycle oils: Because of its special polymers, Spectro 4 maintains its viscosity, whereas the shearing action of motorcycle gears quickly reduces the viscosity of automotive oils.

We've all heard it a thousand times before. Our transmissions are the culprits that force us to buy special, $6-a-quart motorcycle oil instead of the 99 cent special at Pep Boys. We hate to have to do it, but we all know that it's true--or is it?

The question begged an answer, so MCN went looking for evidence that motorcycle oils really are more shear-stable than their automotive counterparts.

Help From the Scientific Quarter

About the same time we began looking into the oil viscosity retention question, we received a letter from John Woolum. a professor of physics at California State University - and a motorcyclist - who noted that he was investigating in the same area on his own. Not being ones to look a gift horse in the mouth, we contacted Dr. Woolum and encouraged him to expand his research on our behalf.

Later in this article Dr. Woolum explains the laboratory procedures he used to generate the statistics used in this article. but for the mean-time let's just take a look at the bottom line when five popular oils (three automotive and two motorcycle) were compared for relative viscosity retention after use in the same motorcycle. (See Figure 2)

As can be seen from the figures, the best-performing oil of the group tested was Mobil 1 automotive oil, a fully synthetic product. In today's market, virtually all oils sold are to some extent para-synthetic, since even standard petroleum products usually contain at least some synthetic-derived additives. However, for the sake of simplicity in this article we have listed the products as petroleum if the primary components are from basic petroleum stock. Those listed as synthetics have their primary components derived from basic synthetic stocks, and may or may not contain any additives derived from petroleum products.

Preliminary Conclusions

The results of these tests seem to support some of the long-standing theories about oils while casting serious doubt on others. Going by these tests it would seem logical to assume that:

1.The viscosity of synthetic-based oils generally drops more slowly than that of petroleum-based oils in the same application.

2.Comparing these figures to viscosity retention for the same oils when used in an automobile (see later text by Prof. Woolum) would indicate that motorcycles are indeed harder on oils than cars.

3.The fastest and most significant drop in the viscosity of petroleum-based oils used in motorcycles occurs during the first 800 miles (or less) of use.
All of these results (1-3) agree with everything the oil companies have been telling us all along. However, the same test data also indicates that:

4.The viscosity of petroleum-based oils, whether designed for auto or motorcycle application, drop at approximately the same rate when used in a motorcycle.

5.There is no evidence that motorcycle-specific oils out-perform their automotive counterparts in viscosity retention when used in a motorcycle.

These last two results (4-5) definitely do not agree with what the motorcycle oil producers have been telling us. In fact the test results not only indicate the two motorcycle oils being outperformed in viscosity retention by the two automotive synthetic products. but even by the relatively inexpensive Castrol GTX, which is a petroleum product. This directly contradicts the advertising claims made by the motorcycle oil producers.

The Oil Companies Reply

At Spectro Oils we talked to three different company spokesmen, all of whom were helpful and provided us with a great deal of information about their products. Unfortunately, despite our repeated requests for the testing data on which their advertising claims were based, the 15 pages of "Lubrication Data" they supplied us contained nothing that could not be found in their regular advertising and marketing packages. No verifiable testing data has been forthcoming.

The Spectro spokesmen were not pleased when informed of our test results, but when pressed, none could come up with a valid reason why their product should have scored the lowest, either. The only comment we got was, "We only wish you had tested our Golden Spectro synthetic instead of the petroleum-based Spectro 4."

Undoubtedly the Golden Spectro would have outscored the regular Spectro in our tests, though how well in comparison to the Mobil 1 and Castrol products we can only guess at this point.

When asked why the Spectro 4 petroleum product sold for $5.00 a quart when comparable automotive oils could be found at less than $1.50 a quart, a Spectro spokesman insisted theirs was "a superior, premium petroleum product, with expensive, shear-stable additives that should outperform automotive oils." That being the case, it should have been the perfect product for our testing.

We made a half-dozen calls to several different divisions within American Honda, but could find no one willing to make any statement regarding their HP4 motorcycle oil. All of the Honda employees we reached were friendly, and tried to help as much as they could, but you must keep in mind that Honda is a huge conglomerate and sometimes the person with the right answers to a question is difficult to track down through the corporate maze. Their Accessories Product Management Division noted that they had a lubrication expert that might be able to help us, but also that he was out of the country on vacation for the next month and could not be reached before this article went to press. Should someone from Honda wish to comment at a later date, we will certainly make room in a later issue.

Spokesmen at both Mobil and Castrol were a bit surprised at our questions, since neither makes any claims for their products in a motorcycling context. However, when we explained the test results, neither company spokesman seemed the least bit surprised, both noting that automotive oils in general had made a quantum leap in viscosity retention technology in the past five or six years. Both companies claimed to be using the very latest in shear-stable polymers for viscosity retention, and while claiming no knowledge of the motorcycle-specific oils' formula, expressed serious doubt that they could contain some type of additive that was superior in this context to that already being used in their automotive oils. Our test results support their assertion.

THE TEST

As we noted earlier, the viscosity-retention figures reported in the table were the result of a series of tests conducted by Dr. John C. Woolum, Professor of Physics at California State University. Since the validity of these tests is likely to be called into question by motorcycle oil marketers, following are Dr. Woolum's lab notes and explanations of the procedures he followed.

Relative Viscosity Retention Comparisons Among Five Brands of Automotive and Motorcycle Oils

by John C. Woolum/ Ph.D.
Professor of Physics
California State University, Los Angeles

The central dogma of motorcycle oil manufacturers and distributors has always been that motorcycles put different demands on their lubricants than do automobiles. In particular, they point to the facts that motorcycles run at higher temperatures and use the same oil in their transmissions as in their engines. The transmission gears supposedly put extreme pressures on the oil molecules, thus causing the long oil polymers to break down. High temperatures can have the same basic effect, as well as additional effects such as the increase in oxidation products.

When the size of the oil polymers decreases ("cut up by the transmission gears," as at least one manufacturer claims), the oil thins. In other words, its viscosity decreases, as well as its ability to lubricate properly. For example, what started out as a 40-weight oil could effectively become a 30-weight oil, or even a 20-weight, after prolonged use. What this means, effectively, is that if the claims of the motorcycle oil producers are valid, they can easily be verified through measurement of viscosity changes on various oils as they are used in different applications.

Measuring the viscosity drop in oils did not seem like too difficult a task, especially since measuring viscosity of solutions of large molecules is a common practice in many biophysics laboratories - mine included. My lab had all the correct equipment - in fact the viscometers that I normally used for solutions of DNA and proteins were originally designed for oil measurements.

Setting the Stage

Viscosity is a measure of the friction between two layers of a liquid sliding relative to one another. It is usually measured in poise, or grams per centimeter per second (g/cm. sec). The basic principle of many viscometers is to measure the time required for a known amount of a liquid to pass through a capillary tube under gravitational force. The time taken will depend on the viscosity and the density of the liquid. The more viscous or less dense the liquid. the longer the time it will take to flow through the capillary.

Therefore in reality, this kind of viscometer does not measure viscosity directly, but rather the ratio of the viscosity to the density of the liquid being tested. This ratio is called the kinematic viscosity. and the common unit for expressing it is in stokes or poise cm^3/gram.

The viscometer used for my measurements was an Ostwald-type, Cannon-Fenske 200, designed to measure kinematic viscosity in the range of 10 to 100 centistokes (a centistoke is one-hundredth of a stoke). The oils being measured had kinematic viscosity between about 10 and 25 centistokes.

For the test samples, I decided to use two types of oils designed specifically for motorcycles and three types of fairly standard automotive oil.

The automotive oils were Castrol GTX 10W40 (petroleum based, $1.24/qt.), Castrol Syntec 10W40 (synthetic, $3.99/qt.) and Mobil 1 15W50 (synthetic, $3.48/qt.). The motorcycle oils were Spectro 4 10W40 (petroleum based, $4.99/qt.) and Honda HP4 10W40 (petroleum/synthetic blend, $5.99/qt.).

Each of these oils was run in the same motorcycles 1984 Honda V65 Sabre-under as near to identical conditions as possible. The oils were sampled for testing at 0, 800 and 1500 miles each.

As temperature has a strong effect on viscosity, I had to make certain it was carefully controlled for the experiments. Using a laboratory temperature control chamber, all measurements were made at 99 degrees Celsius (error factor of plus or minus 0.5 degrees), which is about 210 degrees Fahrenheit. This is the most common temperature used for oil viscosity measurements. It usually took about 15 minutes for each sample to achieve equilibrium within the chamber.

Each oil's kinematic viscosity was compared with its own kinematic viscosity at 0 miles to establish the viscosity ratio. In addition, measurements were made of each oil's density at each state of the tests. The densities were found to change by less than one percent, which is about the limit of the accuracy of the measurements. Therefore, a ratio of the times taken for the oils to pass through the viscometer effectively gives the ratio of their actual viscosity, since the densities cancel out.

What this all means in layman's terms then, is that the ratio established for each oil at the end of each test is a percentage of the amount of original viscosity retained at that point. For example. the Castol GTX sample at 800 miles showed a relative viscosity of 0.722, meaning it had retained 72.2 percent of its original viscosity. Or, if you want to look at it the other way, the Castrol had lost 27.8 percent of its viscosity after 800 miles of use in the motorcycle.

Just for comparison sake, I also tested the viscosity drop of the Castrol GTX automotive oil after use in a 1987 Honda Accord automobile. At 3600 miles of use, the Castrol GTX showed a relative viscosity of 91.8 percent.

As the Mobil 1 had retained so much of its viscosity after the 1500 mile test, it was the only oil I allowed to run longer in the motorcycle. After 2500 miles, the Mobil 1 recorded a relative viscosity of 79.1 percent.

Also, it is worthy of note that from a testing standpoint, the two most similar oils were the Castrol GTX automotive oil and the Spectro 4 motorcycle oil. By similar, I mean that they tested as having almost the same absolute kinematic viscosity and density right out of the container. So starting out as equals, the Castrol maintained its viscosity several percentage points higher than the Spectro, under the same use in the same motorcycle yet the Spectro costs about four times the price of the Castrol.

The Error Factor

As a scientist, I must always ask myself. Are there possible errors in these measurements that would make them invalid? One possibility here would be that there was more particulate matter (contaminants) in some oil samples than in others, which would increase the viscosity numbers of that oil. Particulates disrupt the streamline flow and so increase the viscosity. (Einstein was the first to derive the quantitive expression for the increase in viscosity due to spherically, shaped particles.)

Large particulates should have been removed by the oil filter, and a new filter was used for each test. Still, to determine the effect of smaller particulates the oil samples were centrifuged at 11,000 g (11,000 times the acceleration of gravity) for a period of 10 minutes. A considerable amount of particulate matter was found and removed in all of the 800 mile and 1500 mile samples. However, the change in viscosity made by eliminating these particulates was found to be negligible.

Another possible source of error would be that the conditions to which the oils were subjected were different. In all cases, the distances were comprised of approximately 70 percent city riding and 30 percent freeway riding. The range of temperatures and the average ambient temperature during which the motorcycle was ridden were approximately the same. If anything, the average ambient temperature was higher during the operation of the motorcycle with the Mobil 1 oil, which should have put it at a disadvantage, yet it scored the highest overall in the viscosity retention tests.

Of course the motorcycle did age somewhat during the testing period, which took place over a year-long span. It registered about 4000 miles at the beginning of these tests and about 14,000 at the end. The order in which the oils were tested was: 1) Castrol, 2) Spectro, 3) Mobil and 4) Honda.

Other Criteria

The motorcycle oil producers have suggested that other criteria. such as the amount of wear metals and contaminants, might be unacceptable when using automotive oil in a motorcycle. To test this theory, I sent a sample of the Castrol GTX at 1500 miles to SpectroTech. Inc., for a complete oil analysis. Their findings were that all contaminants (water, dirt, coolant and sludge) were normal.

SpectroTech also reported that all wear elements (antimony, titanium, silver, copper, lead, tin, aluminum, nickel, chromium, cadmium, sodium and boron) were normal except for iron, which was reported as "mildly above normal" at 51 parts per million.

SpectroTech lists acceptable levels for all of the above listed metals except iron, for which they state, "values vary greatly with systems and parts." so it is not clear what exactly is meant by "mildly above normal." Perhaps it was in comparison to cars with 1500 miles on the oil. Also, this could have been due to cam wear, since the early Honda V-4s were known for excessive cam and rocker arm wear.

In any case, again I could find nothing to support the argument that automotive oils were somehow less effective than motorcycle-specific lubricants when used in a motorcycle.

Bottom Line

It could appear from this data, then, that there is no validity to the constantly-used argument that motorcycle-specific oils provide superior lubrication to automotive oils when used in a motorcycle. If the viscosity drop is the only criterion, then there is certainly no reason to spend the extra money on oil specifically designed for motorcycles. There does, however, appear to be a legitimate argument for using synthetic and synthetic-blend oils over the petroleum based products.

MCN's Conclusions

In speaking to a number of people involved in the production, marketing and distribution of motorcycle-specific oils, we could not find anyone who could present a valid argument for discrediting the testing done by Dr. Woolum. In general, they all tried to turn the conversation another direction by bringing up other possible advantages to using their products, while ignoring the viscosity-retention question. Yet without exception it is their own advertising that consistently brings the subject up, touting the special shear-stable polymers as the primary reason motorcyclists should purchase their products.

It is this practice to which we take exception, as we have been unable to find evidence to support these claims. In short, it seems to be nothing more than a clever marketing ploy designed to enhance their products' image and separate motorcyclists from their money.

MCN is ready to print any research or test results provided by the oil companies to support their claims of superior viscosity retention, with this one proviso: The comparisons must be against actual, SG-rated oil products that can be purchased off the shelf at the average auto parts store. Tests against generic, basic-stock mineral oil or against the lower-rated SE and SF oils would lack any credibility in a real-world context.

Despite more than six months of research, reading all the claims and counter-claims printed by dozens of industry experts and lubrication experts, MCN cannot and does not purport to know all there is to know about the differences between automotive and motorcycle oils. However, what we do know is that we can find no substantive evidence that using a high-quality, name-brand automotive oil in an average street motorcycle is in any way harmful or less effective in providing proper lubrication and protection than using the more expensive, motorcycle-specific oils.