Motor Oil Grades Like 5W-30, Explained
In 5W-30, the 5W rates how the oil flows cold (W is for winter) and the 30 how it flows at engine temperature; use the grade your manual lists.
By Damien VolkmanContributing writer.
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The short answer
A motor oil grade describes viscosity: how thick the oil is, or how much it resists flowing. The grading system comes from SAE, the engineering society, and U.S. Lubricants explains it in its guide to oil grades. In each part, a lower number means thinner oil that flows more easily.
What does 5W-30 mean
Read a grade in two parts:
| Part | What it rates | In 5W-30 |
|---|---|---|
| Number before the W | Flow when cold, as at startup | 5W: flows readily on a cold start |
| W | Winter | Marks the cold rating |
| Number after the W | Flow at normal engine temperature (100°C) | 30: thickness once warm |
Oil thickens as it cools and thins as it heats, so a grade with two numbers gives a rating for each condition. U.S. Lubricants notes the trade-off: thinner oil protects better at cold starts, while thicker oil keeps a stronger film against wear when hot.
SAE sets six winter classes (0W, 5W, 10W, 15W, 20W and 25W) and eight hot classes, from 8 to 60. U.S. Lubricants calls 5W-30 the most common grade today, because it suits a wide range of vehicles and engines.
5W-20 vs 5W-30
The two share the same cold rating, so both flow alike on a cold start. The difference shows once the engine is warm: 5W-20 is thinner at operating temperature, and 5W-30 is thicker.
Which is right is not a matter of taste. The carmaker picks the grade for the engine, and U.S. Lubricants notes that newer engines often call for lower-viscosity oils. Using the wrong one costs fuel: the U.S. government's FuelEconomy.gov says running 5W-30 in an engine designed for 5W-20 can lower gas mileage by 1% to 1.5%, and 10W-30 in a 5W-30 engine by 1% to 2%.
Two easy rules:
- Use the grade in your owner's manual.
- Look for "Energy Conserving" in the API symbol on the bottle, which FuelEconomy.gov says marks oil with friction-reducing additives.
How often to change synthetic oil
There is no one mileage that fits every car. The interval for your engine is set by its maker and printed in the owner's maintenance schedule, and it applies to the oil the manual specifies. How you drive matters too: U.S. Lubricants lists the type of driving, the weather and the engine type among the things that change when oil needs replacing, and suggests used oil analysis as a way to set intervals for fleets.
If you switch to a synthetic oil, keep to the grade the manual lists and follow its schedule unless the carmaker says otherwise.
Oils on Wiki Shopping
Wiki Shopping lists 84 oils on October 26, 2026. Of those, 49 are motor oils, 14 hydraulic oils and 9 gear oils, so check the type before you buy. By container size, 40 come as 1-quart bottles and 7 as 5-quart jugs. Browse more in automotive parts and accessories.
How we count
Each oil is grouped by the type and container size its store listing states, from a read-only count of our listing pages on October 26, 2026. Oils that state no type are not in a group, and the counts change as listings come and go.