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Choosing the best motor oil is a topic that comes up frequently in discussions between motorheads, 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 the measure of how thick an oil
is. 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
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 Winter use.
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 best.
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 lowchange, 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 thermal breakdown.
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
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 ashcontent 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 <.5 --- Castrol GTX 122 440 -15 .85 .12 Exxon High Performance 119 419 -13 .70 .11 Havoline Formula 3 125 465 -30 1.0 --- Kendall GT-1 129 390 -25 1.0 .16 Pennzoil GT Perf. 120 460 -10 .9 --- Quaker State Dlx. 155 430 -25 .9 --- Red Line 150 503 -49 --- --- Shell Truck Guard 130 450 -15 1.0 .15 Spectro Golden 4 174 440 -35 --- .15 Spectro Golden M.G. 174 440 -35 --- .13 Unocal 121 432 -11 .74 .12 Valvoline All Climate 125 430 -10 1.0 .11 Valvoline Turbo 140 440 -10 .99 .13 Valvoline Race 140 425 -10 1.2 .20 Valvoline Synthetic 146 465 -40 <1.5 .12 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 Quaker State Synquest 173 457 -76 --- --- Pennzoil Performax 176 --- -69 --- --- 5W-40 Havoline 170 450 -40 1.4 --- 15W-40 AMSOIL 135 460 -38 <.5 --- Castrol 134 415 -15 1.3 .14 Chevron Delo 400 136 421 -27 1.0 --- Exxon XD3 --- 417 -11 .9 .14 Exxon XD3 Extra 135 399 -11 .95 .13 Kendall GT-1 135 410 -25 1.0 .16 Mystic JT8 142 440 -20 1.7 .15 Red Line 149 495 -40 --- --- Shell Rotella w/XLA 146 410 -25 1.0 .13 Valvoline All Fleet 140 --- -10 1.0 .15 Valvoline Turbo 140 420 -10 .99 .13 10W-30 AMSOIL 142 480 -70 <.5 --- Castrol GTX 140 415 -33 .85 .12 Chevron Supreme 150 401 -26 .96 .11 Exxon Superflo Hi Perf 135 392 -22 .70 .11 Exxon Superflo Supreme 133 400 -31 .85 .13 Havoline Formula 3 139 430 -30 1.0 --- Kendall GT-1 139 390 -25 1.0 .16 Mobil 1 160 450 -65 --- --- Pennzoil PLZ Turbo 140 410 -27 1.0 --- Quaker State 156 410 -30 .9 --- Red Line 139 475 -40 --- --- Shell Fire and Ice 155 410 -35 .9 .12 Shell Super 2000 155 410 -35 1.0 .13 Shell Truck Guard 155 405 -35 1.0 .15 Spectro Golden M.G. 175 405 -40 --- --- Unocal Super 153 428 -33 .92 .12 Valvoline All Climate 130 410 -26 1.0 .11 Valvoline Turbo 135 410 -26 .99 .13 Valvoline Race 130 410 -26 1.2 .20 Valvoline Synthetic 140 450 -40 <1.5 .12 5W-30 AMSOIL 168 480 -76 <.5 --- Castrol GTX 156 400 -35 .80 .12 Chevron Supreme 202? 354 -46 .96 .11 Chevron Supreme Synth. 165 446 -72 1.1 .12 Exxon Superflow HP 148 392 -22 .70 .11 Havoline Formula 3 158 420 -40 1.0 --- Mobil 1 165 445 -65 --- --- Mystic JT8 161 390 -25 .95 .1 Quaker State 165 405 -35 .9 --- Red Line 151 455 -49 --- --- Shell Fire and Ice 167 405 -35 .9 .12 Unocal 151 414 -33 .81 .12 Valvoline All Climate 135 405 -40 1.0 .11 Valvoline Turbo 158 405 -40 .99 .13 Valvoline Synthetic 160 435 -40 <1.5 .12
All of the oils above meet current SG/CD ratings and all vehicle manufacturer'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 informed choice.
Any of the ester based synthetics (AMSOIL, Mobil 1, and Syntec), will give
you the benefits that Castrol is making a big deal of in their
advertising. The ability to cling to metal walls is due to the polar
nature of the ester base stock, not something unique to
The Syntec seems to be compromised by it's wide viscosity range. Notice
that the pour point is for all practical purposes, no better than the Mobil 1
15W-50. (actually, it's not as good) While, meeting the viscosity
parameters, the wide range is probably for marketing purposes. The Mobil 1
15W-50 will pump at -
-35 degrees F, which is as good as some conventional 5W-30 oils.
Any of the ester based synthetics (AMSOIL, Mobil 1, and Syntec), will give you the benefits that Castrol is making a big deal of in their advertising. The ability to cling to metal walls is due to the polar nature of the ester base stock, not something unique to Castrol's formulation.
The Data: (add to your current article)
Brand and Weight VI Flash Pour %ash %zinc
Syntec 5W-50 180 437 -49 1.2 0.10
Mobil 1 5W-30 165 445 -65 --- --- 10W-30 160 450 -65 --- --- 15W-50 170 470 -55 --- ---
This page last updated April 27, 2001. Send us your feedback, and come join the Imperial Mailing List - Online Car Club