More Than You Ever Wanted to Know About Motor Oil
by Ed Hackett (email@example.com)
This information appears to have been posted in a
public message to rec.motorcycles. I have not been able to verify the
accuracy or source for this file.
Choosing the best motor oil is a topic that comes up frequently in
discussions between motoheads, whether they are talking about motorcycles
or cars. The following article is intended to help you make a choice based
on more than the advertising hype.
Oil companies provide data on their oils most often referred to as
“typical inspection data”. This is an average of the actual physical and a
few common chemical properties of their oils. This information is
available to the public through their distributors or by writing or
calling the company directly. I have compiled a list of the most popular,
premium oils so that a ready comparison can be made. If your favorite oil
is not on the list get the data from the distributor and use what I have
as a data base.
This article is going to look at six of the most important properties of a
motor oil readily available to the public: viscosity, viscosity index
(VI), flash point, pour point, % sulfated ash, and % zinc.
Viscosity is a measure of the “flowability” of an oil. More specifically,
it is the property of an oil to develop and maintain a certain amount of
shearing stress dependent on flow, and then to offer continued resistance
to flow. Thicker oils generally have a higher viscosity, and thinner oils
a lower viscosity. This is the most important property for an engine. An
oil with too low a viscosity can shear and loose film strength at high
temperatures. An oil with too high a viscosity may not pump to the proper
parts at low temperatures and the film may tear at high rpm.
The weights given on oils are arbitrary numbers assigned by the S.A.E.
(Society of Automotive Engineers). These numbers correspond to “real”
viscosity, as measured by several accepted techniques. These measurements
are taken at specific temperatures. Oils that fall into a certain range
are designated 5, 10, 20, 30, 40, 50 by the S.A.E. The W means the oil
meets specifications for viscosity at 0 F and is therefore suitable for
The following chart shows the relationship of “real” viscosity to their
S.A.E. assigned numbers. The relationship of gear oils to engine oils is
_______________________________________________________________ | | | SAE Gear Viscosity Number | | ________________________________________________________ | | |75W |80W |85W| 90 | 140 | | | |____|_____|___|______________|________________________| | | | | SAE Crank Case Viscosity Number | | ____________________________ | | |10| 20 | 30 | 40 | 50 | | | |__|_____|____|_____|______| | ______________________________________________________________ 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 viscosity cSt @ 100 degrees C
Multi viscosity oils work like this: Polymers are added to a light base
(5W, 10W, 20W), which prevent the oil from thinning as much as it warms
up. At cold temperatures the polymers are coiled up and allow the oil to
flow as their low numbers indicate. As the oil warms up the polymers begin
to unwind into long chains that prevent the oil from thinning as much as
it normally would. The result is that at 100 degrees C the oil has thinned
only as much as the higher viscosity number indicates. Another way of
looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that
will not thin more than a 50 weight would when hot.
Multi viscosity oils are one of the great improvements in oils, but they
should be chosen wisely. Always use a multi grade with the narrowest span
of viscosity that is appropriate for the temperatures you are going to
encounter. In the winter base your decision on the lowest temperature you
will encounter, in the summer, the highest temperature you expect. The
polymers can shear and burn forming deposits that can cause ring sticking
and other problems. 10W-40 and 5W-30 require a lot of polymers (synthetics
excluded) to achieve that range. This has caused problems in diesel
engines, but fewer polymers are better for all engines. The wide viscosity
range oils, in general, are more prone to viscosity and thermal breakdown
due to the high polymer content. It is the oil that lubricates, not the
additives. Oils that can do their job with the fewest additives are the
Very few manufactures recommend 10W-40 any more, and some threaten to void
warranties if it is used. It was not included in this article for that
reason. 20W-50 is the same 30 point spread, but because it starts with a
heavier base it requires less viscosity index improvers (polymers) to do
the job. AMSOIL can formulate their 10W-30 and 15W-40 with no viscosity
index improvers but uses some in the 10W-40 and 5W-30. Mobil 1 uses no
viscosity improvers in their 5W-30, and I assume the new 10W-30. Follow
your manufacturer’s recommendations as to which weights are appropriate
for your vehicle.
Viscosity Index is an empirical number indicating the rate of change in
viscosity of an oil within a given temperature range. Higher numbers
indicate a low change, lower numbers indicate a relatively large change.
The higher the number the better. This is one major property of an oil
that keeps your bearings happy. These numbers can only be compared within
a viscosity range. It is not an indication of how well the oil resists
Flash point is the temperature at which an oil gives off vapors that can
be ignited with a flame held over the oil. The lower the flash point the
greater tendency for the oil to suffer vaporization loss at high
temperatures and to burn off on hot cylinder walls and pistons. The flash
point can be an indicator of the quality of the base stock used. The
higher the flash point the better. 400 F is the minimum to prevent
possible high consumption. Flash point is in degrees F.
Pour point is 5 degrees F above the point at which a chilled oil shows no
movement at the surface for 5 seconds when inclined. This measurement is
especially important for oils used in the winter. A borderline pumping
temperature is given by some manufacturers. This is the temperature at
which the oil will pump and maintain adequate oil pressure. This was not
given by a lot of the manufacturers, but seems to be about 20 degrees F
above the pour point. The lower the pour point the better. Pour point is
in degrees F.
% sulfated ash is how much solid material is left when the oil burns. A
high ash content will tend to form more sludge and deposits in the engine.
Low ash content also seems to promote long valve life. Look for oils with
a low ash content.
% zinc is the amount of zinc used as an extreme pressure, anti- wear
additive. The zinc is only used when there is actual metal to metal
contact in the engine. Hopefully the oil will do its job and this will
rarely occur, but if it does, the zinc compounds react with the metal to
prevent scuffing and wear. A level of .11% is enough to protect an
automobile engine for the extended oil drain interval, under normal use.
Those of you with high revving, air cooled motorcycles or turbo charged
cars or bikes might want to look at the oils with the higher zinc content.
More doesn’t give you better protection, it gives you longer protection if
the rate of metal to metal contact is abnormally high. High zinc content
can lead to deposit formation and plug fouling.
Listed alphabetically — indicates the data was not available
Brand VI Flash Pour %ash %zinc 20W-50 AMSOIL 136 482 -38 20W-40 Castrol Multi-Grade 110 440 -15 .85 .12 Quaker State 121 415 -15 .9 --- 15W-50 Chevron 204 415 -18 .96 .11 Mobil 1 170 470 -55 --- --- Mystic JT8 144 420 -20 1.7 .15 Red Line 152 503 -49 --- --- 5W-50 Castrol Syntec 180 437 -45 1.2 .10 .095 % Phosphor Quaker State Synquest 173 457 -76 --- --- Pennzoil Performax 176 --- -69 --- --- 5W-40 Havoline 170 450 -40 1.4 --- 15W-40 AMSOIL 135 460 -3810W-30 AMSOIL 142 480 -705W-30 AMSOIL 168 480 -76
All of the oils above meet current SG/CD ratings and all vehicle
manufacture’s warranty requirements in the proper viscosity. All are “good
enough”, but those with the better numbers are icing on the cake.
The synthetics offer the only truly significant differences, due to their
superior high temperature oxidation resistance, high film strength, very
low tendency to form deposits, stable viscosity base, and low temperature
flow characteristics. Synthetics are superior lubricants compared to
traditional petroleum oils. You will have to decide if their high cost is
justified in your application.
The extended oil drain intervals given by the vehicle manufacturers
(typically 7500 miles) and synthetic oil companies (up to 25,000 miles)
are for what is called normal service. Normal service is defined as the
engine at normal operating temperature, at highway speeds, and in a dust
free environment. Stop and go, city driving, trips of less than 10 miles,
or extreme heat or cold puts the oil change interval into the severe
service category, which is 3000 miles for most vehicles. Synthetics can be
run two to three times the mileage of petroleum oils with no problems.
They do not react to combustion and combustion by-products to the extent
that the dead dinosaur juice does. The longer drain intervals possible
help take the bite out of the higher cost of the synthetics. If your car
or bike is still under warranty you will have to stick to the recommended
drain intervals. These are set for petroleum oils and the manufacturers
make no official allowance for the use of synthetics.
Oil additives should not be used. The oil companies have gone to great
lengths to develop an additive package that meets the vehicle’s
requirements. Some of these additives are synergistic, that is the effect
of two additives together is greater than the effect of each acting
separately. If you add anything to the oil you may upset this balance and
prevent the oil from performing to specification.
The numbers above are not, by any means, all there is to determining what
makes a top quality oil. The exact base stock used, the type, quality, and
quantity of additives used are very important. The given data combined
with the manufacturer’s claims, your personal experience, and the
reputation of the oil among others who use it should help you make an