Motor Oil Explained: A Beginner’s Guide to Grades and Types

Motor oil is a blend of base oils and additives that lubricates, cleans, cools, seals and protects your engine. Its viscosity grade, such as 5W-30, tells you how it flows cold and at operating temperature, and its API, ILSAC or OEM specification tells you which engines it suits. Synthetic oils use engineered base oils for stronger protection in extreme heat and cold. This guide covers what motor oil is, what it does and how grades and types differ.

What is motor oil?

Put simply, motor oil is the lubricant that prevents metal components inside your engine from touching and tearing each other apart or welding themselves together.

Motor oil reduces friction and heat and helps keep your engine clean. Without motor oil, your engine would destroy itself in a matter of seconds.

What’s in motor oil?

Motor oil contains two fundamental components: base oils and additives.

The ingredients in motor oil formulations work together to produce the final product that you put in your engine. Think of coffee as an analogy, where the base oil is water and the additive package is coffee beans.

Base oils comprise the largest portion of the oil. They lubricate internal moving parts, absorb heat and seal the piston rings.

Motor oil base oils can be composed of conventional petroleum, chemically synthesized materials or a combination of synthetics and petroleum (called semi-synthetic or synthetic blend).

Petroleum (or conventional) base oils are refined from crude oil. Contaminating elements such as sulfur, nitrogen, oxygen and metal components such as nickel or vanadium are inherent to crude oil and cannot be completely removed through the refining process. The oil refining process separates the various types of molecules in the oil by weight, leaving molecules similar in weight but dissimilar in structure, reducing performance.

Synthetic base oils, on the other hand, are highly engineered to include only beneficial molecules. As such, they contain no contaminants or molecules that don’t serve a designed purpose. Their versatility and pure, uniform molecular structures impart properties that provide better friction-reduction, optimum fuel efficiency, maximum film strength and extreme-temperature performance conventional lubricants just can’t touch.

Check out this post for the definitive guide on synthetic vs. conventional oil.

Motor oil additives

The various chemicals that comprise a motor oil’s additive system provide anti-wear, anti-foam, anti-corrosion, acid-neutralization, viscosity-maintenance, detergency and dispersancy properties.

A few examples of chemical additives include zinc, phosphorus and boron. Striking the perfect balance of the proper additives in relation to the base oil is a tough balancing act for oil formulators, especially as vehicles grow more demanding and sophisticated.

What does motor oil do?

Motor oil does far more than lubricate. It performs 12 functions:

Oil forms a film that keeps metal parts apart. Less friction means less wasted power and less heat, so the engine runs better and lasts longer.

Where parts touch, they wear. A strong oil film and antiwear additives help bearings, cam lobes, cylinder walls and gears hold their factory clearances.

Reducing friction minimizes heat in moving parts. Oil also absorbs heat from hot surfaces and carries it to a place where it can safely dissipate, such as the oil sump.

Motor oil has no natural ability to resist rust and corrosion; those properties come from additives that neutralize corrosive acids or form a barrier between metal and corrosive material.

Detergent additives keep deposits from forming on hot parts such as pistons and piston rings, and the base oil’s solvency helps dissolve contaminants.

Dispersant additives keep soot, dirt and wear particles suspended in the oil so they can’t clump into sludge. The filter traps some; the rest drain out at the next oil change.

Motor oil acts as a dynamic seal at the piston ring/cylinder interface, keeping combustion gases in the combustion chamber, which maximizes horsepower and helps keep hot gases from contaminating the oil in the sump.

Because motor oil is nearly incompressible, it transfers force, such as when it actuates hydraulic valve lifters or variable valve timing.

A strong lubricant film resists rupture and absorbs sudden spikes in force, spreading them over a broader contact area to minimize wear and damage.

A full oil film between parts quiets the engine. Lifter tick at startup and gear whine are often signs that parts aren’t fully separated.

Less friction and the right viscosity mean less energy wasted. That’s why many new engines specify thinner oils for fuel economy, provided the oil is strong enough to protect.

This is the sum of the other 11. Oil that controls friction, wear, heat, deposits and corrosion helps an engine run at peak performance and last longer.

For how each function works, and how the familiar “seven functions of motor oil” map to these 12, read What Does Motor Oil Do? The 12 Functions of a Lubricant.

What is viscosity?

Viscosity refers to the oil’s resistance to flow and is the most important property of an oil. The viscosity of oil varies with changes in temperature – thinner when hot, thicker when cold.

Although oil must flow at cold temperatures to lubricate the engine at startup, it must also remain thick enough to protect the engine at high operating temperatures. When an oil is used at a variety of temperatures, as with most engines, the change in viscosity should be as minimal as possible.

Wouldn’t it be handy to have a number that indicated the oil’s viscosity change? We do, and it’s called the Viscosity Index (VI). It is measured by comparing the viscosity of the oil at 40°C (104°F) with its viscosity at 100°C (212°F). The higher the VI, the less the viscosity changes with temperature changes and the better the oil protects the engine. Synthetics typically have a higher VI than conventional oils.

For a deep dive on motor oil viscosity and how it affects engine protection, see this post.

SAE viscosity grades (oil weights)

The Society of Automotive Engineers (SAE) has developed several viscosity classifications or grades, such as 5W-30, 10W-40 and 15W-50. People often refer to them as “oil weights.”

These viscosity grades (or oil weights) designate the specific ranges in which the particular oil falls.

The “W” indicates it is suitable for use in cold temperatures. (Think of the “W” as meaning “Winter.”) The classifications increase numerically; the lower the number, the lower the temperature at which the oil can be used for safe and effective engine protection. Higher numbers reflect better protection for high-heat and high-load situations.

The overwhelming majority of oils today are multi-viscosity, meaning they behave differently at different operating temperatures to provide the best of both worlds – good cold-flow when the temperature drops and reliable protection once the engine reaches operating temperature.

For example, a 5W-30 motor oil performs like an SAE 5W at 40ºC (104ºF) and an SAE 30 motor oil at 100ºC (212ºF).

How is gasoline motor oil classified?

The American Petroleum Institute (API) developed a classification system to identify oils formulated to meet the different operating requirements of gasoline and diesel engines.

The API system has two general categories: S-series and C-series.

The S-series service classification emphasizes oil properties critical to gasoline engines.

When an oil passes a series of both bench tests and engine tests (API Sequence tests), it can be sold bearing the applicable API service classification.

The classifications progress alphabetically as the level of lubricant performance increases. Each classification replaces those before it. Oils meeting the latest API classification, API SQ, may be used in any engine calling for it or a previous API specification unless otherwise specified.

The API SP category is the most recent classification, replacing those before it. SP oils are designed to provide…

• • Improved oxidation resistance

• • Deposit protection
  • Maximum fuel economy
  • Emissions-system performance
  • Resistance to a new type of engine knock called low-speed pre-ignition (LSPI)

How is diesel engine oil classified?

CK- and FA-series classifications pertain to diesel engines and include those shown below.

Note the FA-4 classification was introduced primarily to help maximize fuel economy in over-the-road trucks and pertains only to some 2017 and newer diesel engines

2027 Changes

The American Petroleum Institute (API) has approved the next diesel oil categories, which replace CK‑4 and FA‑4 starting Jan. 1, 2027:

• API CL‑4 — replaces CK‑4 as the backward‑compatible oil for existing engines, in today’s familiar viscosity grades.
• API FB‑4 — lower‑viscosity oils for newer diesel engines designed to use them, for improved fuel economy.

Improvements target better oxidation control, enhanced wear protection, better seal compatibility, protection for emissions aftertreatment systems and fuel economy gains.

Putting it all together

That’s a ton of information, but it boils down to this: motor oil is a vital part of your engine’s longevity and performance.

In the long run, it pays to use the best oil specified for your vehicle. The initial price of a high-quality synthetic motor oil may be more, but huge savings can be realized through improved engine performance and longevity.

More AMSOIL motor oil guides

How motor oil works

• What Does Motor Oil Do? The 12 Functions of a Lubricant

Specifications

• Engine Oil Specs Explained: API, ILSAC and dexos

Synthetic vs. conventional

• Synthetic vs. Conventional Oil: The Differences That Matter
• Are all Synthetic Oil Groups the Same? Group III vs IV vs V
• 100% Synthetic Oil vs Full Synthetic Oil: What’s the Difference?
• Synthetic Blend vs. Full Synthetic Oil: Pros and Cons
• How Do I Switch from Conventional to Synthetic Oil?
• Can I Mix Synthetic and Conventional Oil?
• Does Synthetic Oil Cause Leaks?
• Synthetic Oil Beats Conventional in AAA Test
• Is Motor Oil Made from Natural Gas Better?
• Do Oil Additives Work? What the Testing Shows
• How is Motor Oil Tested?
• The History of Synthetic Oil (and AMSOIL)

Viscosity

• Understanding Oil Viscosity
• 5W-20 vs. 5W-30 vs. 10W-30: What’s the Difference?
• What Happens If I Use a Thicker Oil in My Engine?
• Should I Use a Lighter Winter Oil Weight?
• All You Need to Know About Oil Cold Flow

Oil changes and intervals

• AMSOIL Oil Change Interval
• Do I Need to Change Oil Every 3,000 Miles?
• Oil Life Monitors – Everything You Need To Know
• The DIY Oil Change: Avoid These 5 Common Mistakes
• Why Do Motorists Change Oil in the Spring and Fall?
• Seasonal Maintenance Guide: Spring, Summer, Fall and Winter
• Save Time With ATV/UTV Oil Change Kits
• How To Dispose of Used Motor Oil
• What is the Shelf Life of Motor Oil and Grease?
• Engine Oil Analysis: How to Take a Sample and Read the Results

Filters and filtration

• Can You Reuse an Oil Filter? Your Questions Answered
• Oil Filter Basics: Efficiency, Capacity and Flow
• AMSOIL Ea Oil Filters
• Bypass Oil Filters: How an Oil Bypass System Works
• How to Install an AMSOIL Bypass Filter System
• How to Change Your Car’s Air Filter
• How to Change a Cabin Air Filter
• Fuel Filters Require Regular Service

Sludge, oil consumption and engine health

• How Engine Sludge Forms. And How to Prevent It.
• Synthetic Lubricants Resist Sludge Better than Conventional Oils
• No Sludge. Just Protection.
• Why is My Car’s Engine Burning Oil?
• Low Oil Pressure Symptoms and Causes
• Is an Engine Flush Good or Bad?

Recommended AMSOIL products

Start with AMSOIL synthetic motor oil and find the exact AMSOIL products for all your vehicles with the AMSOIL Product Guide.