Surprising New Evidence on
the Viscosity-Retention Question
article is from the February, 1994 Motorcycle Consumer News in
the article "Motorcycle Oils vs. Automotive Oils".
Full credit for this article and study go to the original
Walk into any motorcycle dealership parts
department and you are virtually guaranteed to see prominent displays of
oils produced specifically for use in motorcycle engines. Since dealers
are not about to waste valuable floor or counter space on a product unless
it produces a decent profit, it is obvious that motorcycle-specific oils
have become one of the premier parts department cash cows of the 1990s.
Of course advances in lubrication technology have resulted in some fairly
expensive premium, synthetic and synthetic-blend products for automobiles
also. But as you can see from our pricing research at a half-dozen auto
parts and cycle parts stores, the average purchase price for the
motorcycle-specific lubricants runs about 120 percent higher for petroleum
products and 185 percent higher for synthetic products than do their
automotive counterparts. (See Figure 1)
The companies marketing these high-priced motorcycle lubricants would have
us believe that their products are so superior to the automotive oils as
to justify paying two and three times the price. But are we really getting
the added protection promised when we purchase these products? MCN decided
to look beyond the advertising-hype, specifically to see if the claims of
prolonged and superior viscosity retention could be verified. What we
found may very well change your mind about what should go into your
motorcycle's crankcase in the future.
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
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
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
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
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.
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
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
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.
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
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
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
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,
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
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.
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.
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.
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
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.
Petroleum Based, Multiple Viscosity, SG-Rated,
Best Retail Prices Found
Maxum 4 Premium
Average Price Differential: 319.5%
Synthetic Based and Petroleum/Synthetic
Multiple Viscosity, SG-Rated Oils
Best Retail Prices Found
Golden Spectro 4
Maxum 4 Extra
Pep Boys Synthetic
Average Price Differential: 185.0%
|Relative Viscosity Retention
(as a percentage of initial viscosity retained
after normal use in the same motorcycle)