If you’ve ever looked at a bottle of motor oil, you’ve probably seen numbers such as 0W-20, 5W-30, 10W-40, or 15W-50. These numbers may seem confusing at first, but they play a crucial role in choosing the right oil for your vehicle. Engine oil numbers indicate the oil’s viscosity, or how easily it flows, at both cold and hot temperatures.
Engine Oil Numbers Mean
Engine oil numbers describe the viscosity grade established by the Society of Automotive Engineers (SAE).
For example:
- 0W-20
- 5W-30
- 10W-40
Each number has a specific meaning.
- The first number (before the W) indicates cold-weather performance.
- The W stands for Winter.
- The second number indicates the oil’s viscosity at normal engine operating temperature (100°C or 212°F).
Simply put:
- Lower first numbers flow better during cold starts.
- Higher second numbers provide a thicker protective film when the engine gets hot.
Related Engine Oil Viscosity Chart(Comprehensive Guide)
Understanding the First Number
The first number tells you how well the oil flows when the engine is cold.
Examples:
A lower number means:
- Easier cold starts
- Faster oil circulation
- Less engine wear immediately after starting
Cold starts are when most engine wear occurs because oil has not yet fully circulated.
What Does the “W” Mean?
Many people think the “W” stands for Weight, but that’s incorrect.
The W means Winter.
It tells you that the oil has been tested for low-temperature performance.
Winter-rated oils remain fluid in cold temperatures, allowing them to protect the engine immediately after startup.
Related Engine Oil Density Chart(For All Oil Types)
Understanding the Second Number
The second number indicates how thick the oil remains once the engine reaches operating temperature.
Common examples include:
- 20
- 30
- 40
- 50
- 60
Higher numbers indicate thicker oil at high temperatures.
For example:
| Oil Grade | Hot Temperature Thickness |
|---|---|
| SAE 20 | Thin |
| SAE 30 | Medium |
| SAE 40 | Thick |
| SAE 50 | Very Thick |
| SAE 60 | Extremely Thick |
Thicker oil can provide:
- Better protection under heavy loads
- Improved protection in hot weather
- Increased film strength
However, oil that is too thick may reduce fuel economy and circulate more slowly.
Related SAE Engine Oil Viscosity Chart(For All Grades)
What Does 0W-8 Mean?
A 0W-8 oil:
- Flows exceptionally well in extremely cold temperatures
- Offers the lowest viscosity for maximum fuel efficiency
- Becomes SAE 8 when the engine reaches operating temperature
It’s often used in:
- Some hybrid vehicles
- New-generation fuel-efficient engines
- Vehicles specifically designed for ultra-low viscosity oil
What Does 0W-16 Mean?
A 0W-16 oil:
- Flows extremely well during cold starts
- Reduces engine friction
- Becomes SAE 16 when hot
It’s often used in:
- Modern Toyota vehicles
- Honda vehicles requiring 0W-16
- Hybrid cars
- Fuel-efficient gasoline engines
What Does 0W-20 Mean?
A 0W-20 oil:
- Flows extremely well in freezing temperatures
- Provides excellent fuel economy
- Becomes SAE 20 when hot
It’s often used in:
- Most modern passenger vehicles
- Hybrid vehicles
- Compact SUVs
- Fuel-efficient gasoline engines
Related 0W vs 5W(Difference and Which One Should You Use)
What Does 0W-30 Mean?
A 0W-30 oil:
- Provides excellent cold-weather protection
- Offers slightly thicker protection than 0W-20 when hot
- Becomes SAE 30 at operating temperature
It’s often used in:
- European vehicles
- Luxury sedans
- Turbocharged engines
- Cold-climate driving
What Does 0W-40 Mean?
A 0W-40 oil:
- Starts flowing quickly in freezing temperatures
- Maintains excellent protection under high engine temperatures
- Becomes SAE 40 when hot
It’s often used in:
- Performance cars
- Sports cars
- European luxury vehicles
- Turbocharged engines
What Does 5W-20 Mean?
A 5W-20 oil:
- Flows well during cold starts
- Improves fuel economy
- Becomes SAE 20 when hot
It’s often used in:
- Ford vehicles
- Honda vehicles
- Chrysler vehicles
- Daily commuter cars
Related Engine Oil Comparison Chart(Which Brand Takes the Crown)
What Does 5W-30 Mean?
A 5W-30 oil:
- Performs well in cold weather
- Provides balanced engine protection
- Becomes SAE 30 when hot
It’s often used in:
- Most gasoline-powered vehicles
- SUVs
- Pickup trucks
- Family sedans
What Does 5W-40 Mean?
A 5W-40 oil:
- Flows easily during cold starts
- Provides stronger high-temperature protection
- Becomes SAE 40 when hot
It’s often used in:
- Turbocharged engines
- Diesel vehicles
- European vehicles
- High-performance cars
What Does 5W-50 Mean?
A 5W-50 oil:
- Performs well in cold weather
- Maintains a thick protective film under heavy loads
- Becomes SAE 50 when hot
It’s often used in:
- Racing engines
- High-performance sports cars
- Modified vehicles
- Heavy-duty gasoline engines
Related Engine Oil Thickness Chart(A to Z Guide)
What Does 10W-30 Mean?
A 10W-30 oil:
- Performs well in moderate climates
- Offers reliable cold-start protection
- Becomes SAE 30 when hot
It’s often used in:
- Older passenger vehicles
- Light trucks
- Small engines
- Lawn and garden equipment
What Does 10W-40 Mean?
A 10W-40 oil:
- Performs well in moderate climates
- Starts slightly thicker than 5W oil
- Maintains SAE 40 protection at operating temperature
It’s often used in:
- Older engines
- High-mileage vehicles
- Performance cars
- Motorcycles
What Does 10W-50 Mean?
A 10W-50 oil:
- Provides reliable cold-start performance
- Offers excellent protection under high engine temperatures
- Becomes SAE 50 when hot
It’s often used in:
- Performance motorcycles
- Racing vehicles
- Sports cars
- Heavy-duty gasoline engines
Related Engine Oil Boiling Point(for All Engine Oil Types)
What Does 10W-60 Mean?
A 10W-60 oil:
- Flows well in moderate temperatures
- Maintains an extremely thick protective film when hot
- Becomes SAE 60 at operating temperature
It’s often used in:
- High-performance sports cars
- Track-day vehicles
- Racing engines
- High-output European engines
What Does 15W-30 Mean?
A 15W-30 oil:
- Performs well in warm climates
- Offers moderate high-temperature protection
- Becomes SAE 30 when hot
It’s often used in:
- Agricultural equipment
- Diesel engines
- Industrial machinery
- Utility vehicles
What Does 15W-40 Mean?
A 15W-40 oil:
- Performs best in warm climates
- Provides excellent protection under heavy loads
- Becomes SAE 40 when hot
It’s often used in:
- Diesel pickup trucks
- Commercial trucks
- Agricultural equipment
- Construction machinery
What Does 15W-50 Mean?
A 15W-50 oil:
- Offers dependable performance in warmer weather
- Provides excellent high-temperature protection
- Becomes SAE 50 when hot
It’s often used in:
- High-mileage engines
- Performance vehicles
- Air-cooled motorcycles
- Classic cars
What Does 20W-20 Mean?
A 20W-20 oil:
- Is designed for consistently warm climates
- Provides stable lubrication at operating temperature
- Becomes SAE 20 when hot
It’s often used in:
- Older engines
- Agricultural equipment
- Industrial machinery
- Vintage vehicles
What Does 20W-40 Mean?
A 20W-40 oil:
- Performs best in warm weather
- Provides thicker protection than SAE 30 oils
- Becomes SAE 40 when hot
It’s often used in:
- Older passenger vehicles
- Motorcycles
- Utility vehicles
- High-mileage engines
What Does 20W-50 Mean?
A 20W-50 oil:
- Is thicker during cold starts than lower winter grades
- Provides maximum protection in hot conditions
- Becomes SAE 50 when hot
It’s often used in:
- Classic vehicles
- High-mileage engines
- Older motorcycles
- Engines operating in very hot climates
What Does 25W-50 Mean?
A 25W-50 oil:
- Is intended for very warm operating conditions
- Maintains a thick oil film under heavy loads
- Becomes SAE 50 when hot
It’s often used in:
- Heavy-duty diesel engines
- Industrial equipment
- Off-road machinery
- Older high-displacement engines
What Does 25W-60 Mean?
A 25W-60 oil:
- Is designed for consistently warm environments
- Provides one of the thickest oil films available
- Becomes SAE 60 when hot
It’s often used in:
- Racing engines
- High-performance V8 engines
- Worn or rebuilt engines with larger clearances
- Extreme high-temperature applications
Single-Grade vs Multi-Grade Oils
Single-Grade Oil
Examples include:
- SAE 30
- SAE 40
- SAE 50
These oils have only one viscosity rating.
Advantages:
- Simple formulation
- Often used in small engines
- Good for constant temperatures
Disadvantages:
- Poor cold-weather performance
- Less versatile
Multi-Grade Oil
Examples include:
- 0W-20
- 5W-30
- 5W-40
- 10W-30
Advantages:
- Excellent all-season performance
- Better cold starts
- Good protection when hot
- Improved fuel economy
Today, nearly all passenger vehicles use multi-grade oils.
Related Best Oil for Air Cooled Engines(5 Best Picks 2026)
Common Engine Oil Grades Explained
| Oil Grade | Best Use |
|---|---|
| 0W-16 | Hybrid vehicles |
| 0W-20 | Modern fuel-efficient engines |
| 5W-20 | Ford, Honda, Chrysler vehicles |
| 5W-30 | Most gasoline engines |
| 5W-40 | Turbocharged engines |
| 10W-30 | Older passenger cars |
| 10W-40 | Warm climates and older engines |
| 15W-40 | Diesel trucks |
| 20W-50 | High-mileage or classic vehicles |
Why Modern Cars Use Lower Oil Numbers
Many new vehicles recommend:
- 0W-16
- 0W-20
- 5W-20
Manufacturers use thinner oils because they:
- Reduce engine friction
- Improve fuel economy
- Lower emissions
- Circulate faster
- Meet stricter environmental regulations
Engine designs have also become more precise, allowing thinner oils to provide sufficient protection.
Why Older Engines May Need Thicker Oil
Older engines often develop:
- Larger internal clearances
- Worn bearings
- Increased oil consumption
A thicker oil like 10W-40 or 20W-50 may:
- Reduce oil burning
- Improve oil pressure
- Decrease engine noise
- Provide better sealing
However, always follow your owner’s manual unless a mechanic recommends otherwise.
Do Higher Numbers Mean Better Oil?
No.
Many people mistakenly believe:
- 10W-40 is always better than 5W-30.
- 20W-50 protects every engine better.
This is false.
The best oil is simply the one recommended by the manufacturer.
Using oil that’s too thick or too thin can lead to:
- Reduced fuel economy
- Poor lubrication
- Increased engine wear
- Lower performance
How Manufacturers Choose Oil Numbers
Automakers test engines under thousands of conditions, including:
- Cold starts
- Highway driving
- Heavy towing
- High-speed operation
- Stop-and-go traffic
- Hot climates
- Freezing weather
They select the viscosity that provides the best balance between:
- Protection
- Fuel economy
- Performance
- Longevity
Always check your owner’s manual before choosing motor oil.
Does Climate Affect Oil Numbers?
Absolutely.
Cold Climates
Choose lower winter numbers such as:
- 0W-20
- 5W-30
These oils flow quickly during freezing temperatures.
Hot Climates
Higher operating-temperature numbers may offer better protection, such as:
- 5W-40
- 10W-40
- 15W-40
These maintain a stronger oil film in extreme heat.
Engine Oil Numbers and Fuel Economy
Thinner oils generally improve fuel economy because they reduce internal engine resistance.
For example:
| Oil Grade | Fuel Economy |
|---|---|
| 0W-16 | Excellent |
| 0W-20 | Excellent |
| 5W-20 | Very Good |
| 5W-30 | Good |
| 10W-40 | Moderate |
| 20W-50 | Lowest |
This is one reason manufacturers increasingly recommend lower-viscosity oils for modern vehicles.
Can You Switch Between Oil Grades?
Sometimes, yes.
For example:
- Switching from 5W-30 to 0W-30 may be acceptable in colder climates.
- Switching from 5W-30 to 10W-30 may be suitable in warmer conditions.
However, changing viscosity should only be done if it aligns with the manufacturer’s recommendations or is advised by a qualified mechanic. Using an incorrect grade can affect lubrication, performance, and warranty coverage.
FAQs
Is 5W-30 thicker than 0W-20?
Yes. At operating temperature, 5W-30 is thicker than 0W-20. It also has a slightly higher cold viscosity.
Does 0W mean the oil freezes?
No. It means the oil performs exceptionally well in cold temperatures and remains fluid enough for reliable engine lubrication during startup.
Can I use 10W-40 instead of 5W-30?
Only if your vehicle manufacturer specifically approves both viscosity grades. Otherwise, using a thicker oil than recommended may reduce fuel economy and affect engine performance.
What happens if I use the wrong oil numbers?
Using the wrong viscosity can cause poor lubrication, increased engine wear, reduced fuel efficiency, difficult cold starts, or inadequate protection under high temperatures.
Which oil number is best?
There is no universal “best” oil grade. The correct choice depends on your vehicle’s engine design, operating conditions, climate, and the manufacturer’s recommendations.
Final Thoughts
Engine oil numbers are more than just labels, they provide essential information about how motor oil performs in different temperatures. The first number and the “W” indicate how well the oil flows during cold starts, while the second number shows how thick it remains when the engine is at normal operating temperature. Choosing the correct viscosity ensures proper lubrication, minimizes wear, improves fuel economy, and helps maximize engine life.
