More Than You Ever Wanted to Know About Motor Oil

by Ed Hackett (


Editor’s Note:

This information appears to have been posted in a

public message to 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

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. 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

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


AMSOIL                  136     482     -38     20W-40

Castrol Multi-Grade     110     440     -15     .85     .12

Quaker State            121     415     -15     .9      ---


Chevron                204      415     -18     .96     .11

Mobil 1                170      470     -55     ---     ---

Mystic JT8             144      420     -20     1.7     .15

Red Line               152      503     -49     ---     ---


Castrol Syntec         180      437     -45     1.2     .10 	.095 % Phosphor

Quaker State Synquest  173      457     -76     ---     ---

Pennzoil Performax     176      ---     -69     ---     ---


Havoline               170      450     -40     1.4     ---


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

informed choice.