Oil viscosity numbers describe how motor oil performs at low and high temperatures. In a grade such as 5W-30, 5W is the winter viscosity grade, and 30 is the higher-temperature viscosity grade.
The numbers do not indicate oil quality, and a higher number is not automatically better. Your engine is designed to use a specific viscosity or range of approved viscosities. Always follow the vehicle manufacturer’s recommendations.
At Tire Discounters, we verify the required viscosity, performance specifications, oil capacity, and filter for your vehicle before beginning an oil change.
What Does 5W-30 Mean?
A multigrade oil such as 5W-30 is designed to meet two sets of viscosity requirements.
- 5W identifies how the oil performs in specified cold-temperature cranking and pumping tests.
- W stands for winter, not weight.
- 30 identifies the oil’s higher-temperature viscosity grade.
Motor oil naturally becomes thicker as it cools and thinner as it heats. Multigrade oil is formulated to provide suitable flow during cold starts while maintaining the higher-temperature viscosity required by the engine.
The numbers are classification grades, not literal measurements. A 5W-30 oil does not become a “5-weight” oil in winter and a “30-weight” oil in summer.
What Does the W Mean in Motor Oil?
The W means winter.
The number before the W represents the oil’s winter viscosity grade. A lower winter grade qualifies under colder laboratory test conditions.
For example, a 0W oil meets lower-temperature cranking and pumping requirements than a 5W oil. That does not mean every vehicle should use 0W oil. Use the winter grade approved by the vehicle manufacturer rather than automatically choosing the lowest number available.
Cold-temperature performance matters because oil must begin circulating through bearings, camshafts, timing components, and turbochargers after the engine starts.
What Does the Second Number Mean?
The second number represents the oil’s higher-temperature SAE viscosity grade.
In 5W-30, the second number is 30. In 0W-20, it is 20.
A 30-grade oil meets different high-temperature viscosity requirements than a 20-grade oil. However, a higher number is not automatically more protective.
Modern engines are designed around specific:
- Bearing clearances
- Oil-pump output
- Oil-flow rates
- Turbocharger requirements
- Variable-valve-timing systems
- Emissions and fuel-economy targets
The manufacturer selects a viscosity that allows these systems to operate as intended. Using oil outside the approved range may affect cold-start circulation, oil pressure, variable valve timing, fuel economy, or high-temperature protection.
What Does SAE Mean on an Oil Bottle?
SAE refers to SAE International, historically known as the Society of Automotive Engineers.
SAE publishes the J300 engine-oil viscosity classification used for grades such as 0W-20, 5W-30, and 5W-40.1
An SAE grade identifies viscosity. It does not confirm that the oil meets every engine-performance requirement.
Other labels and approvals may identify performance standards related to wear, deposits, oxidation, emissions-system compatibility, and other engine requirements. These may include:
- API service categories
- ILSAC standards
- GM dexos specifications
- European ACEA sequences
- Manufacturer-specific approvals
SAE tells you the viscosity grade. API, ILSAC, and manufacturer approvals help tell you whether the oil meets the additional requirements for your engine.2
0W-20 vs. 5W-30: What Is the Difference?
The two oils have different winter and higher-temperature grades.
| Oil Grade | Winter Grade | Higher-Temperature Grade |
|---|---|---|
| 0W-20 | 0W | 20 |
| 5W-20 | 5W | 20 |
| 5W-30 | 5W | 30 |
| 10W-30 | 10W | 30 |
| 5W-40 | 5W | 40 |
For example:
- 0W-20 and 5W-20 share the same higher-temperature grade but have different winter grades.
- 5W-20 and 5W-30 share the same winter grade but have different higher-temperature grades.
- 0W-20 and 5W-30 differ in both portions of the classification.
These oils should not be treated as interchangeable unless the vehicle manufacturer approves both grades.
Can I Use a Different Oil Viscosity?
Only use an alternative viscosity when the vehicle manufacturer permits it.
Some owner’s manuals list more than one approved grade for certain temperatures or operating conditions. Others specify one viscosity for normal operation.
Do not switch to a thicker oil simply because:
- The vehicle has high mileage
- You live in a warm climate
- You tow or haul
- A thicker oil seems more protective
Likewise, do not switch to a thinner oil solely to improve fuel economy or cold-weather performance.
A different viscosity may change how oil moves through the engine and how systems such as variable valve timing or hydraulic lifters operate. If your engine leaks, burns oil, or has low oil pressure, changing viscosity is not a substitute for diagnosing the underlying condition.
Does Synthetic Oil Have a Different Viscosity?
Synthetic, conventional, and synthetic-blend oils can all be produced in common viscosity grades.
For example, both conventional and full synthetic oils may be available as 5W-30. The viscosity grade tells you how the oil behaves in SAE viscosity tests. It does not tell you what type of base oil was used.
Full synthetic oils generally provide more stable performance across temperature extremes than conventional oils, but switching oil types does not change the viscosity your engine requires.
A vehicle that requires 5W-30 still needs an approved 5W-30 product unless the manufacturer permits another grade.
How Do I Find the Right Oil for My Vehicle?
Start with the owner’s manual. Look for:
- The required SAE viscosity
- The required API or ILSAC standard
- Any manufacturer-specific approval
- Instructions for different temperatures or driving conditions
- The recommended oil-change interval
The oil-fill cap may also display a viscosity, but the owner’s manual provides more complete information.
Current oil standards may be backward-compatible with earlier requirements, but compatibility depends on the vehicle and specification. API recommends following the vehicle manufacturer’s viscosity and performance recommendations.2
What If My Oil Level Is Low?
Use a product that meets the viscosity and performance requirements listed by the manufacturer.
Using the same oil already in the engine is preferable when it is available. In an urgent situation, matching the required viscosity and specifications is more important than matching the brand name.
If the oil level repeatedly drops between services, the vehicle may have a leak or an oil-consumption concern. Adding oil restores the level but does not identify or repair the cause.
Follow the owner’s manual for the correct oil-checking procedure and avoid overfilling the engine.
How Tire Discounters Selects the Right Oil
Tire Discounters does not select motor oil based only on vehicle age, climate, or the number on the bottle.
Before service, we verify:
- Vehicle and engine
- Required oil viscosity
- Applicable performance specifications
- Oil capacity
- Correct oil-filter application
Tire Discounters offers full synthetic, conventional, and heavy-duty diesel oil changes. Premium Valvoline full synthetic is the standard starting point for gasoline vehicles, but we may use another appropriate oil type or product when the vehicle requires it.3
If your vehicle needs additional oil, a specialty filter, or a manufacturer-specific product, we explain the requirement and any price difference before completing the service.
Frequently Asked Questions
Is 5W-30 thicker than 0W-20?
At higher test temperatures, a 30-grade oil meets a higher viscosity range than a 20-grade oil. Their winter grades also differ. Use the grade approved for your engine rather than choosing based only on which oil appears thicker.
Does the W mean weight?
No. The W stands for winter and identifies the oil’s low-temperature viscosity classification.
Is a lower first number better?
A lower winter grade qualifies at lower cold-test temperatures, but it is only appropriate when approved by the vehicle manufacturer.
Can high-mileage vehicles use thinner oil?
High mileage alone does not determine viscosity. Use a manufacturer-approved grade. High-mileage formulations are available in several viscosities and are different from simply choosing thicker oil.
Can I mix different viscosities?
Avoid mixing viscosities unless necessary for an urgent top-off. Use a product meeting the manufacturer’s requirements and return to the correct, consistent product at the next service.
The Bottom Line
Oil viscosity numbers describe how motor oil performs under specified low- and high-temperature conditions.
In 5W-30:
- 5W is the winter viscosity grade.
- W stands for winter.
- 30 is the higher-temperature viscosity grade.
The best viscosity is not the highest, lowest, or most expensive grade. It is the viscosity approved for your specific engine.
Schedule an oil change with Tire Discounters, and our team will verify the correct oil, filter, and quantity for your vehicle.
Sources Cited
1. SAE International. Engine Oil Viscosity Classification, SAE J300. Defines the engine-oil viscosity classifications used for grades such as 0W-20 and 5W-30.
2. American Petroleum Institute. Motor Oil Guide. Explains oil viscosity and performance labeling and advises drivers to follow vehicle-manufacturer recommendations.
3. Tire Discounters. Oil Change Service. Describes Tire Discounters’ full synthetic, conventional, and heavy-duty diesel oil services and vehicle-specific oil selection.
