Everything you wanted to know about Motor Oil,
but were afraid to ask
By Ed Hackett (edh@wheeler.wrc.unr.edu) and Steve Andersen (andersen@me.udel.edu)

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 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.

|                                                             |
|      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. 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. 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 by weight and then alphabetically, "---" indicates the data was not available

Brand                    VI    Flash    Pour    %Ash   %Zinc
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      ---
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

Brand                    VI    Flash    Pour    %Ash   %Zinc
Castrol Multi-Grade     110     440     -15     .85     .12
Quaker State            121     415     -15     .9      ---

Chevron                 204     415     -18     .96     .11
Mobil 1                 180     430     -55     ---     ---
Mystic JT8              144     420     -20     1.7     .15

Brand                    VI    Flash    Pour    %Ash   %Zinc
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
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

Brand                    VI    Flash    Pour    %Ash   %Zinc
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                 ---     430     -60     ---     ---
Pennzoil PLZ Turbo      140     410     -27     1.0     ---
Quaker State            156     410     -30     .9      ---
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

Brand                    VI    Flash    Pour    %Ash   %Zinc
AMSOIL                  168     480     -76     <.5     ---
Castrol GTX             156     400     -35     .80     .12
Chevron Supreme         202?    354     -46     .96     .11
Exxon Superflow HP      148     392     -22     .70     .11
Havoline Formula 3      158     420     -40     1.0     ---
Mobil 1                 150     430     -65     ---     ---
Mystic JT8              161     390     -25     .95     .1
Quaker State            165     405     -35     .9      ---
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
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 more expensive synthetics; AMSOIL, Mobil 1, and Spectro 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.

Types of oil on the the market today:

  1. Paraffins (basically dinosaur created mineral oils)
  2. Polyalfaolefins (a synthetic polymer)
  3. Di-esters (synthetic polymer)
  4. Polyesters
Mobil 1 is made of Polyalfaolefin and has some additives in a Paraffin based carrier oil. The Polyalfaolefin supposedly provides better temperature resistance, and has a natural multi-viscosity nature. Paraffin based oils need additives to get a multi-vis property, therefore supposedly as the oil wears, it begins to lose it multi-viscosity properties (a 20-50 will act like say a straight 30 wt after xxxx miles).

Golden Spectro has 3 components, Paraffins (for additive carrier), Polyalfaolefin, and Di-ester. The Di-ester has even greater temp resistance than PAOlefin. Also supposedly (according to Motul dude) the Esters have a property which makes them "stick" to surfaces and create a film under non-dynamic conditions. Normal dinosaur oils and bearings require motion to create and maintain the oil film. The Esters have a property by which they create a film even in a static state. What this effectively means is that even at start-up or after long (indefinite) periods of storage there will be an oil film on the critical wear surfaces of the engine (and we all know that this is when most engine wear occurs).

Motul's 2100 series is a mixture of Paraffin and Di-Ester, their new 300 series (?) is totally synthetic Di-ester. Note that the Motul dude said that Golden Spectro was an excellent oil and could not say his was better or not except that the 2100 was somewhat cheaper. Also I will not get into a discussion of additive packages including anti-wear, friction modifiers, and extreme pressure additives... He also said that the 300 series oil (100% synthetic) was overkill for a street bike ( even high perf.) he said that it was only worthwhile in a racing environment. Good thing since it's $12 / liter!!!!!

Redline oil is a Polyester based oil (dunno about blends). Apparently the Polyester is just as good as Di-esters, except for one thing: Polyesters are not necessarily compatible with the non-metallic parts of the engine. It has been known to cause seals to swell, etc. It is for this reason that supposedly it is not useable in aircraft, and also I don't think that it is rated (SF, SG, etc) since all rated oils must pass a gasket compatibility test...