Viscosity is how easily oil flows. Thin oil pours faster; thick oil pours slower. Engines need oil that flows well at start-up and still protects parts once everything is hot. Multi-grade oils, the kind most vehicles use today, are designed to do both.
Reading a grade like 0W-20
Motor oil grades follow the SAE J300 viscosity classification. A label such as 0W-20 has two parts:
- The number before the W (0W, 5W, 10W) describes cold-temperature flow. The W stands for winter. A lower number means the oil is graded to flow more easily when it’s cold, which helps at start-up.
- The number after the W (20, 30, 40) describes viscosity at operating temperature, measured at 100°C (212°F) under the SAE test. A higher number means a thicker grade at that hot temperature.
So 0W-20 is graded for easy cold flow (0W) and a 20-weight thickness when hot. A 5W-30 is a bit thicker when hot (30) and uses a 5W cold grade. Single-grade oils such as SAE 30 have no W number and are graded only at the hot temperature.
What the grade does not tell you
- It is not a quality ranking. A 5W-30 is not “better” than a 0W-20. They are different grades for different engines and climates.
- It is not a license to mix grades freely. Use the grade your owner’s manual lists. Some manuals allow more than one grade depending on temperature.
- It is not the full specification. Manuals also call for industry specs (for example API or ILSAC categories) and sometimes manufacturer specs. Match those as well as the viscosity.
How to choose for your vehicle
- Open the owner’s manual to the recommended lubricants section. That page is the authority.
- Note the viscosity and the specs it lists, including any notes about temperature ranges or severe service.
- Look the vehicle up in AMSOIL’s official lookup guide or the vehicle lookup to see which AMSOIL products are listed for that grade and application.
- Track the change interval with the oil change interval tracker, using the miles or months from the manual or oil label.
Diesel engines, motorcycles and equipment often have their own grades and specs. Use the matching lookup: heavy-duty, motorcycle or all guides.
Frequently asked questions
What does the W in 0W-20 mean?
W stands for winter. The number before the W is the oil’s cold-temperature viscosity grade under SAE J300. A lower number means the oil is graded to flow more easily when cold.
Is a higher second number always better protection?
Not by itself. The second number is the hot-temperature viscosity grade. Engines are designed around a specific grade. Using a thicker or thinner oil than the manual calls for can cause problems.
Can I use 5W-30 if the manual says 0W-20?
Follow the owner’s manual. Some manuals allow alternate grades in certain temperatures; many do not. When in doubt, use the primary grade the manufacturer lists.
Where do I find which AMSOIL viscosity my car takes?
Check the manual first, then use AMSOIL’s vehicle lookup or official lookup guide to see the products listed for your year, make and model.
AMSOIL motor oil and the lookup guide
AMSOIL makes motor oil in many viscosities. One example is AMSOIL Signature Series 0W-20 100% Synthetic Motor Oil. AMSOIL says Signature Series is “formulated for drivers who demand the absolute best engine protection” and is “designed to control wear, resist volatility and maintain viscosity under extreme heat, pressure and cold-start conditions.”
Product wording above is quoted or summarized from amsoil.com. Read each product’s label and data sheet for directions.
The right viscosity is the one your manual specifies. Use AMSOIL’s vehicle lookup or AMSOIL’s official lookup guide rather than guessing from the label alone.