How Is Motor Oil Tested? Understanding Laboratory Oil Tests
A motor oil technical data sheet can contain an intimidating collection of numbers, temperatures, units and ASTM test methods. Kinematic viscosity, viscosity index, HTHS, cold-cranking viscosity, pour point and Total Base Number all describe different characteristics of a lubricant — but no single number tells the whole story.
Laboratory testing allows lubricant properties and performance to be measured under controlled and repeatable conditions. Some tests measure a basic physical property of the oil, while others simulate demanding operating conditions or measure how effectively a lubricant protects actual engine components.
An ASTM test method defines how a particular measurement is performed. This allows results produced by different laboratories and lubricant manufacturers to be compared using a standardised procedure rather than each company developing its own test.
Not All Oil Tests Measure the Same Thing
Oil testing ranges from relatively simple measurements of physical properties through to sophisticated engine tests conducted under precisely controlled operating conditions. Understanding what type of test produced a result is important when interpreting lubricant specifications and performance claims.
Physical Property Tests
Measure characteristics such as viscosity, cold-temperature behaviour, volatility, flash point and alkalinity. These values commonly appear on lubricant technical data sheets.
Bench Performance Tests
Use specialised laboratory equipment to evaluate properties such as wear protection, oxidation resistance, deposits, corrosion or friction under controlled conditions.
Engine Tests
Run lubricants in specified engines under controlled operating cycles to evaluate real engine phenomena such as wear, deposits, sludge, oxidation and low-speed pre-ignition.
Field Testing
Evaluates lubricant performance in vehicles or equipment operating in real service, often over substantial mileage or operating hours.
Common Motor Oil Laboratory Tests
Many of the measurements found in AMSOIL technical data sheets are produced using standard ASTM methods. Each test answers a different question about how the lubricant behaves.
Kinematic Viscosity
Measures how readily an oil flows under gravity at a specified temperature. Engine-oil technical data commonly reports kinematic viscosity at 40°C and 100°C in centistokes (cSt).
Helps answer: How thick is the oil at this temperature?
Viscosity Index
Calculates a Viscosity Index from kinematic-viscosity measurements, typically at 40°C and 100°C. A higher VI indicates that viscosity changes less as temperature changes.
Helps answer: How sensitive is the oil’s viscosity to temperature?
High-Temperature High-Shear Viscosity
Measures apparent viscosity at 150°C under extremely high shear. These conditions are designed to represent demanding areas of an operating engine, such as highly loaded bearings.
Helps answer: Does the oil maintain sufficient viscosity under severe heat and shear?
Cold-Cranking Viscosity
Uses a Cold-Cranking Simulator (CCS) to measure apparent viscosity at very low temperatures. The result correlates with the resistance an engine encounters while cranking in cold conditions.
Helps answer: How readily can the engine crank with this oil in extreme cold?
Pour Point
Determines the lowest temperature at which movement of the oil is observed under the specified test conditions. It provides an indication of very-low-temperature fluidity.
Helps answer: At approximately what temperature does the oil cease to flow under this test?
Total Base Number (TBN)
Measures the lubricant’s alkaline reserve. TBN is particularly useful when considering an oil’s ability to neutralise acidic by-products that can develop during service.
Helps answer: How much alkaline reserve does the fresh lubricant contain?
Four-Ball Wear Test
Evaluates wear protection by rotating one steel ball against three stationary balls under controlled load, temperature and speed, then measuring the resulting wear scars.
Helps answer: How effectively does the lubricant control wear under these specific bench-test conditions?
Understanding Kinematic Viscosity
Kinematic viscosity is one of the most commonly published lubricant properties. It is normally expressed in centistokes (cSt), equivalent to mm²/s, and is frequently measured at 40°C and 100°C.
The measurement is particularly useful when comparing the actual viscosity of lubricants that use different classification systems. SAE engine-oil, SAE gear-oil, ISO and AGMA grade numbers are not directly comparable, even where the lubricants have similar physical viscosities.
Our Oil Viscosity Comparison Chart explains how these different viscosity systems relate to one another.
What Does Viscosity Index Tell Us?
Oil viscosity changes with temperature. It becomes more viscous as temperature falls and less viscous as temperature rises. Viscosity Index (VI) provides a convenient way of expressing how strongly an oil’s kinematic viscosity changes with temperature.
ASTM D2270 calculates VI using kinematic-viscosity data at 40°C and 100°C. In general, a higher VI indicates a smaller change in viscosity across that temperature range.
Viscosity Index is therefore not another measurement of how “thick” the oil is. Two lubricants can have similar viscosity at 100°C while having different viscosity indexes and different behaviour as their temperature changes.
What Is HTHS Viscosity?
Kinematic viscosity at 100°C provides useful information, but an operating engine contains areas where oil experiences considerably more severe conditions. Bearings and other highly loaded interfaces can subject the lubricant to both elevated temperature and extremely high shear rates.
High-Temperature High-Shear (HTHS) viscosity is intended to examine lubricant viscosity under conditions more representative of these demanding areas.
ASTM D5481 measures HTHS viscosity at 150°C using a multicell capillary viscometer. HTHS viscosity is an important parameter in many engine-oil specifications because maintaining an adequate lubricant film under high temperature and shear is critical to protecting highly loaded components.
How Is Cold-Temperature Performance Tested?
Cold starting creates a very different lubrication challenge. As temperature falls, oil becomes more viscous and the engine must work harder to turn during starting.
The Cold-Cranking Simulator (CCS) used by ASTM D5293 measures the apparent viscosity of engine oil at low temperatures. The result correlates with low-temperature engine cranking behaviour.
Importantly, cold-cranking viscosity does not describe every aspect of cold-start lubrication. It is specifically related to cranking behaviour rather than predicting the oil’s flow through the oil pump and lubrication system.
This distinction demonstrates why lubricant specifications often use multiple tests to evaluate performance rather than relying on a single viscosity measurement.
Bench Tests vs Engine Tests
A laboratory bench test such as the Four-Ball Wear Test can be extremely useful because conditions can be tightly controlled and repeated. Change the lubricant while keeping the load, temperature, speed and test duration constant, and differences in measured wear can be compared.
However, a bench apparatus is not an internal-combustion engine. An operating engine simultaneously exposes oil to heat, combustion products, fuel dilution, contaminants, aeration, high shear, different materials and many different lubrication regimes.
This is why modern engine-oil specifications also incorporate sophisticated engine sequence tests.
Evaluates areas including high-temperature oxidation, viscosity increase and piston deposits using a controlled engine-test procedure.
Evaluate valvetrain wear under controlled operating conditions, providing a more application-specific assessment than a simple bench wear test.
Evaluates an oil’s contribution to controlling LSPI, an abnormal combustion phenomenon particularly relevant to modern turbocharged direct-injection petrol engines.
Does One Test Prove That One Oil Is Better?
An oil performing exceptionally well in one test does not automatically make it the best lubricant for every engine or application. A properly formulated engine oil must balance many different and sometimes competing performance requirements.
Wear protection is important, but so are oxidation resistance, deposit control, sludge protection, cold-start performance, high-temperature viscosity, fuel economy, emissions-system compatibility, seal compatibility and numerous other characteristics.
This is also why an impressive result from a non-standard demonstration should not automatically be treated as equivalent to meeting an established lubricant specification. Standardised ASTM and engine tests provide defined procedures and conditions that allow meaningful comparisons to be made.
How Testing Relates to Oil Specifications
Performance standards such as API, ILSAC and ACEA do not simply specify a viscosity. They establish combinations of performance requirements intended to demonstrate that an engine oil provides the required protection and behaviour for its intended application.
These requirements can incorporate numerous laboratory and engine tests covering areas such as wear, oxidation, deposits, sludge, fuel economy and low-temperature performance.
For more information about these classifications, see our Engine Oil Standards Guide.
Understanding AMSOIL Technical Specifications
AMSOIL product pages contain technical properties such as kinematic viscosity, Viscosity Index, HTHS viscosity, pour point and TBN. These figures provide useful information about the physical characteristics of each lubricant, but their significance is much easier to understand when you know how each measurement is produced.
As we expand the AMSOIL Knowledge Centre, individual test methods will be covered in greater detail, explaining how each test works, how its results should be interpreted and what the measurement can — and cannot — tell you about lubricant performance.
