Motor Oil Standards and Specifications Explained
In addition to the glossy advertising and marketing materials, most reputable motor oil companies make the results of tests performed on their oils available to the general public. Usually this information is available on downloadable data sheets; and depending on how impressive the numbers are, they may also appear prominently in the companies' advertising materials.
But what do all thosetests and numbers mean?
This page explains some of the most common tests and specifications related to engine oils in what I hope will be simple language. Understanding a bit about these tests will help you choose the best oil for your particular vehicle's engine.
You'll notice that most of the tests mentioned here have an ASTM number associated with them. That refers to the specifications that ASTM International (formerly the American Society for Testing and Materials) has developed for the tests to assure the uniformity and reliability of the tests.
The significance of the ASTM tests for consumers is that test results that cite tests performed using ASTM standardized tests have comparison value. You can look up another oil's results on the same tests and get an idea of the two oils' comparative performance. That's not true for home-grown tests that manufacturers devise on their own. It's not that in-house tests are necessarily invalid, but rather that they're of limited comparison value.
In short, when a lubricant company's marketing literature states that "tests have shown" some wonderful thing about their oil, your first question should be, "What tests?" If it's not a standardized ASTM test, then the results are less useful for comparison purposes.
Let's look at some of the more common specifications and test results cited on motor oil specification and data sheets.
There are many different service rating systems managed by national, regional, and international organizations including ILSAC (worldwide), API (North America), ACEA (Europe), and others. These organizations define standards for oils that can be expressed in simple codes, and manufacturers use those codes to specify which oils should be used in the engines the build.'
Some examples of service ratings include GF-6 (ILSAC); SP, CK4, or FA4 (API); or A3/B3 or A5/B5 (ACEA). The rating corresponding to a motor oil's intended country of sale is usually printed on a prominent seal on the oil's label. Other standards the oil may meet are usually in the fine print.
Total Base Number (TBN) (ASTM D-2896 and ASTM D-4739)
TBN is a measure of an oil's ability to neutralize acids that could harm the engine, and for how long it will be effective at doing so. It's a very important number to consider in selecting an oil.
At a minimum, I want a TBN of 6 in an oil for a gasoline vehicle engine, or 10 for a diesel vehicle engine (unless the manufacturer recommends a higher value). Fortunately, most quality synthetic oils far exceed those TBN numbers.
Flash Point and Fire Point (ASTM D-92)
A motor oil's flash point is that temperature at which its vapors will momentarily catch fire in the presence of a source of ignition. The fire point is the temperature at which its vapors will be able to maintain a continuous flame. Most motor oil data sheets will list both values, which are usually quite close. If the data sheet doesn't list the flash point, the oil's Material Safety Data Sheet should list it.
For most consumers, the flash point is most useful as a very rough measure of an oil's overall quality. If the flash point is lower than 200° C (392° F), I'd look for another oil. High-quality motor oils usually have flash points higher than 220° C (428° F).
Noack Volatility Test (ASTM D-5800)
Frequently (and incorrectly) capitalized as NOACK, the original version of this test was developed by Dr. Kurt Noack in 1936. It involves heating the oil to a temperature of 250° C (482° F) for one hour while blowing air over it. The fraction of oil that evaporates, by weight, is the score. A lower score means that there was less evaporation, so lower is better.
Although it's been around for a long time, the Noack test took on special importance with the advent of GDI (gasoline direct-injection) engines. The less oil that evaporates, the less carbon buildup there will be on the intake valves. In fact, if you own a car with a GDI engine, the Noack score is one of the most important factors you should consider. A Noack score of 10 is about as high as I'd consider using in a GDI engine. A score of 6 or lower would be even better.
If a motor oil's data sheet doesn't list the Noack score, I suggest you look for another oil. The Noack test is mandatory under both API and ACEA standards, so the manufacturer definitely knows how the oil scored. If they're hiding the number, there's a reason.
You can read more about the importance of selecting the proper oil for a GDI engine here.
Thermo-oxidation Engine Oil Simulation Test (TEOST) (ASTM D-6335 and ASTM D-7097)
The TEOST test was jointly developed by Chrysler Corporation and Savant Laboratories to test the deposit-forming tendencies of an oil in turbocharged engines. Both current versions of the test measure the amount of deposits that form on a metal rod in a heated oil sample. The more weight the rod gains, the higher the score; so lower scores are better.
Like the Noack score, the TEOST score is especially important when selecting an oil for GDI engines. I personally consider TEOST scores in the 15mg to 20mg range just barely okay, 8mg to 15mg good, and less than 8mg excellent.
What I'm referring to as "label viscosity" is, well, the viscosity on the label. It's the number in the big typeface. It a number that sums up all the viscosity factors and tests performed on the oil in a simple way that consumers can understand.
Motor oils used in vehicles are almost always multigrade oils whose viscosity is expressed as a hyphenated number such as "5W-30". The first number has a "W" behind it because it refers to the oil's "winter" (cold-weather) viscosity.
If you want to know more about viscosity, I wrote a whole page about it. Otherwise, the main things you need to know about the label viscosity are, firstly, to use oil of a viscosity specified by the vehicle's manufacturer; and secondly, that all of the other motor oil tests and specifications are viscosity-dependent. So if you want to do more research about a particular oil, make sure that the data sheet refers to the brand, line, and viscosity of the oil you're researching.
Kinematic Viscosity (ASTM D-445)
Kinematic viscosity is the ratio of absolute viscosity to the fluid's density at a given temperature. Almost all motor oil data sheets will list the kinematic viscosity values of an oil at 40° C (104° F) and 100° C (212° F). Unless you're an engineer or a serious oil geek, however, you needn't worry about these values. They're mainly important as factors used to determine other, more easily-understood specifications.
Viscosity Index (ASTM D-2270)
The viscosity index, or VI, is a value calculated from the oil's kinematic viscosity values at 40° C (104° F) and 100° C (212° F). It's a measure of the oil's viscosity stability throughout the temperature scale.
Back in olden times, when motor oils were primitive by today's standards, 100 was the maximum possible VI value. Nowadays, 100 is a very realistic minimum acceptable value for a single-viscosity oil. For a multigrade oil, 140 is a perfectly-reasonable minimum to shoot for, and higher is even better.
Engine-Protection and Lubricity Factors
Four-Ball Wear Test (ASTM D-4172)
The Four-Ball Wear Test is a very popular test that pops up in many comparison charts between competing oils. Unfortunately, it's not a very useful test for engine oils. It's more useful for greases and gear oils. Because it's so frequently referenced, however, it's worth explaining.
The way the Four-Ball Wear Test works is that a steel ball is rotated against three other steel balls that are covered with the lubricant being tested. The temperature, pressure, speed, and length of time of the test are all specified in the ASTM standard. When the test has run its course, the "wear scars" on the balls are measured, and the average size is the result. The smaller the average wear scar, the better the oil protected the balls from wear. At least in theory. In practice, the Four-Ball Wear Test simulates ball bearings in a race more closely than it does the inside of an engine, making its usefulness as an engine oil test debatable, at best.
If you choose to use the Four-Ball Wear Test for comparison purposes, a smaller scar is better.
Timken Extreme Pressure Test (ASTM D-2782)
The Timken test was developed in the 1930's by the Timken Company and was first used to test aviation lubricants. It measures a lubricant's resistance to extreme pressure.
The way the Timken test works is that a standardized bearing race is rotated against a standardized block of steel while being lubricated with the oil being tested. The pressure is carefully increased until the test block becomes scored. The highest pressure at which no scoring occurs (or more commonly, an average of the results over several tests) is the oil's "Timken score."
When using the Timken test for comparison purposes, a higher score is better.
Tapered Bearing High Temperature / High Shear Test (HT/HS) (ASTM D-4683)
As its name implies, this test measures an oil's viscosity stability at high temperatures and high stress conditions, using a tapered bearing simulator viscometer at a temperature of 150° C (302° F). Among other factors, how well the polymers used to modify the oil's viscosity hold up under high stress conditions will affect the results of this test.
The results of the HT/HS are measured in centipoise (cP). One cP is 100 poise, which are a unit of measurement in the centigram-gram-second (CGS) measurement system.
From an engine protection perspective, a higher HT/HS score is better; and virtually all quality oils score at least 3.0 in this regard. From a fuel-economy perspective, some argue that a lower score would be better; but realistically, any fuel economy gains from a lower HT/HS would likely be very small.
Cold-Weather Oil Tests and Specifications
Borderline Pumping Temperature (ASTM D-3829)
This is a very useful cold-weather specification for the average consumer because it's easy to understand. Unfortunately, it's missing from many data sheets.
The borderline pumping temperature is simply the lowest temperature at which the oil will be able to flow through and lubricate the engine. To use this specification, just make sure the borderline pumping temperature is well below the coldest temperature you'd ever expect where you live. Nice and simple.
Mini-Rotary Viscometer (MRV) TP-1 Test (ASTM D-4684)
The MRV is a more complex test of an oil's resistance to flow at extremely low temperatures. It attempts to measure how difficult it will be to pump the oil through the engine at startup in cold weather.
The test is performed by heating an oil to normal operating temperature, and then gradually reducing it over a period of at least 45 hours to a final temperature of between -10° C (14° F) and -40° C (-40° F). The test results are usually reported at intervals along the final temperature scale. A lower score (less resistance to flow at a given temperature) is better.
Pour Point (ASTM D-97 or ASTM D-5949)
Pour point is another cold-temperature test, and is simply the lowest temperature at which an oil is able to be poured. ASTM D-97 is a manual method for determining the pour point using a jar, and ASTM D-5949 is an automated method. There also was an ASTM D-2602 standard that is no longer commonly used.
The only thing an average consumer needs to know about the pour point is that it should be well below the coldest temperature likely to occur where they live.
Cold Crank Simulator (CCS) Apparent Viscosity Test (ASTM D-5293)
The CCS is yet another cold-temperature test. It measures the apparent viscosity of the oil within a temperature range of -5° C (23° F) to -35° C (-31° F)at high shear rates. The test is designed to simulate starting an engine in cold temperatures. The results are expressed in centipoise at a given temperature, and lower is better.
A score of 3,500 cP or less at the coldest temperature you can expect where you live would be an excellent value. A score in the 5,000 cP - 6,000 cP neighborhood would be as high as I'd want to go for an oil being used during the winter.