Which 30 Weight Oil Do I Use, 0W-30, 5W-30 or 10W-30?

We get asked this question so often that we have included this special page dedicated exclusively to answering the question.

AMSOIL 0W-30, 5W-30 and 10W-30 synthetic motor oils are ALL 30 weight oils. The answer is that ANY one can be used regardless if your vehicle owners manual says to use, for example, a 5W-30. "W" means winter. In winter weather the 0W oil will flow like a 0W oil, and the 5W will flow like a 5W oil and a 10W will flow like a 10W oil just until the engine warms up. In order to understand the differences one has to first understand that the numerical values given to these various weight oils are strictly empirical numbers. For example, 0W does not mean that the oil has no weight. That is one of the reasons why we say it is strictly an empirical number.

In order to determine the differences between the three oils one has to look at the kinematic viscosity of each lubricant. The kinematic viscosity is essentially the amount of time, in centistokes, that it takes for a specified volume of the lubricant to flow through a fixed diameter orifice at a given temperature.

Let's compare the kinematic viscosity of the three AMSOIL lubricants:

AMSOIL 0W-30 is 57.3 cST @ 40 deg. C, & 11.3 cST @ 100 deg. C

AMSOIL 5W-30 is 59.5 cST @ 40 deg. C, & 11.7 cST @ 100 deg. C

AMSOIL 10W-30 is 66.1 cST @ 40 deg. C, & 11.7 cST @ 100 deg. C.

As you can see from the data above the kinematic viscosities are extremely close. Therefore, whether you use the 0W-30, 5W-30 or the 10W-30 is strictly a matter of choice. With the small differences in kinematic viscosity you would be hard-pressed to detect these differences on initial engine start-up without specialized engine test equipment.

All three oils are excellent motor oils and ANY one can be used in a vehicle which requires either a 0W-30, 5W-30 or 10W-30 oil as well as in several other engine applications including an engine which recommends a 5W- 20 oil.

AMSOIL Signature Series 0W-30 synthetic motor oil is one of the best synthetic lubricants AMSOIL manufacturers for gasoline engine passenger vehicles and light trucks. The molecular and chemical technology used to develop this oil was derived from AMSOIL's Racing Oil. It is a 35,000 mile/1-year motor oil. This is the same oil used by numerous police vehicles and severe duty fleets nationwide. In fact, many national racing teams use the 0W-30 ( as well as AMSOIL Series 2000 20W-50 Racing Oil). The extra horsepower and friction reduction from the 0W-30 often assists a race driver in attaining superior performance. In fact, we know which race teams use it, but cannot disclose that information.

This leads to the next topic: many people also ask us if the 0W-30 is too thin a viscosity oil for high ambient temperature operation. The answer is absolutely not! Thicker viscosity oils are not always necessarily better since in addition to its' various engine lubrication functions, an oil must also effectively transfer heat. Only about 60% of an engines cooling is performed by the engine coolant, and only on the upper half of the engine. The remaining 40% of an engines cooling is performed mainly by the engine oil.

Although a vehicle that is recommended to use a 30 weight oil can also use a 40 weight oil, it is usually not needed. You will gain absolutely no benefit from using a thicker viscosity oil if it is not needed. The only time we recommend a 40 weight oil, such as AMSOIL's 10W-40, to a customer in a passenger car or light truck application is if the vehicle's engine is excessively worn and consumes oil at a higher than normal rate or if the vehicle is being used for very severe duty, high load, high temperature applications.

We hope this brief discussion has answered your questions. If we have not sufficiently answered your questions or if you would like an additional explanation or recommendation for your specific application please email us using the link at the bottom of this page. 

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         More Than You Ever Wanted to Know About Motor Oil
                  By Ed Hackett    edh_at_maxey.unr.edu

     [Ed is a chemical engineer who works for the University of Nevada's Desert        Research facility -- Ed.].

           
       v1.1 Change in description of viscosity.
       v1.2 Updated info on AMSOIL (courtesy of Morgan McArthur   mgn_at_inel.gov)

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 lose 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 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 also shown.

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 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 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 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.

The Data:
Listed alphabetically    --- indicates the data was not available

Brand                             VI    Flash    Pour    %ash   %zinc

20W-50
AMSOIL (old)                   136     482     -38     <.5     ---
AMSOIL (new)                 157     507     -44     ---     ---
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
AMSOIL                            124     500     -49     ---     ---
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 (old)                      135      460     -38     <.5     ---
AMSOIL (new)                    164      462     -49     ---     ---
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 (old)                       142      480     -70     <.5     ---
AMSOIL (new)                     162      520     -76     ---     ---
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 (old)                       168      480     -76     <.5     ---
AMSOIL (new)                     186      464     -76     ---     ---
Castrol GTX                         156      400     -35     .80     .12
Chevron Supreme               202?     354     -46     .96     .11
Chevron Supreme Synt.      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
manufacture's warranty requirements in the proper viscosity. All are "good
enough", but those with the better numbers (LIKE AMSOIL) 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 cost is
justified in your application.

The extended oil drain intervals given by the vehicle manufacturers
(typically 7500 miles) and synthetic oil companies (AMSOIL up to 35,000 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.

--------------------------------------------------------------------------------

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