Short answer
In a multigrade such as 5W-30, the W identifies the winter grade and the second number identifies the high-temperature grade. A lower W grade meets cold-cranking and pumping limits at colder test temperatures. Choose a grade permitted by your manual together with its required performance specification.
What viscosity means
Viscosity is a fluid’s resistance to flow. Oil thins as it heats and thickens as it cools, so an engine needs oil that flows quickly at a cold start and still keeps a strong enough film between moving parts at operating temperature.
Motor-oil viscosity grades are defined by the SAE J300 standard. A grade is a range of measured values, not one exact number, and two oils with the same grade can still differ in formulation.
Reading a multigrade label
A grade such as 5W-30 has two parts. The “5W” is the winter grade: it is based on cold-cranking and cold-pumping tests, and a lower number means the oil meets those limits at a colder test temperature. It is not a direct viscosity measurement at every possible starting temperature.
The second number, “30,” is the grade at engine operating temperature, based on kinematic viscosity at 100 °C and a high-temperature, high-shear measurement at 150 °C. A 40 is thicker at operating temperature than a 30, and a 20 is thinner.
A single-grade oil, such as SAE 30, has only one grade and is typically used in some small engines, older equipment, or specialized applications.
Source: AMSOIL: understanding oil viscosity.
Thicker is not automatically better
Modern engines are designed around a specific viscosity. Variable valve timing, cylinder deactivation, hydraulic tensioners, and tight bearing clearances can depend on the oil the manufacturer specified. A heavier grade than the one allowed can slow cold-start oil flow and may affect those systems or fuel economy.
Very low grades such as 0W-16, 0W-8, or 0W-20 are used where an engine was designed for them. Using one in an engine that calls for a heavier grade is not a safe substitution.
The specification matters as much as the grade
The owner’s manual usually lists both a viscosity and a performance standard, such as an API service category, an ILSAC standard, or a manufacturer approval (for example, dexos, VW 508 00, or MB 229.5). Current gasoline categories include API SQ and ILSAC GF-7, introduced in 2025; manuals for earlier vehicles often list API SP or ILSAC GF-6.
Check that the product lists the specification your engine requires. Matching the viscosity alone does not establish compatibility, especially for European, turbocharged, and diesel engines with emissions equipment.
Source: API Motor Oil Guide: SQ and GF-7 categories.
Choosing a grade for your vehicle
Start with the grade printed in the owner’s manual or on the oil-fill cap. Some manuals list more than one permitted grade with a temperature chart; in that case, choose within those options based on the temperatures you actually see.
Towing, track use, or modifications do not by themselves justify changing grades. If you think your application needs something different, confirm it with the manufacturer, a qualified technician, or the engine builder.
Turn the label into a shopping decision
Record four separate details: vehicle and engine, permitted viscosity, required performance specification or approval, and oil-change quantity with filter. Use those details together in the vehicle lookup; searching only for 5W-30 leaves many unrelated applications in the results.
Distinguish a manufacturer approval from a lubricant supplier’s recommendation for an application. If an approval is required, verify its current status through the product documentation and the applicable manufacturer listing. A matching bottle color or familiar product family is not evidence of that approval.
Keep engine and gear-oil grade numbers separate
Engine-oil and automotive gear-oil grades use different SAE classification systems. Do not compare 75W-90 gear oil with 10W-30 motor oil as if both numbers were points on one scale. The correct component specification determines which type of lubricant belongs there.
For an engine oil change, pair the verified viscosity with the correct filter and interval. For an axle, transmission, or transfer case, start a separate component lookup.
Source: AMSOIL: how to read a gear-oil viscosity chart.
Sources checked . Prices and product guidance may change.
Frequently asked questions
Is 5W-30 thicker than 0W-30 at operating temperature?
No. Both meet the SAE 30 range at operating temperature. The difference is at cold temperature, where the 0W oil meets its cold-flow limits at a lower temperature.
Can I use 5W-30 instead of 5W-20?
Only if your vehicle documentation permits 5W-30. Some manuals list both; many modern engines specify one grade. Check the manual or ask your dealer or technician.
Does W stand for weight?
No. The W stands for winter. It identifies the cold-temperature grade in a multigrade oil.
Should I change viscosity with the seasons?
Only within the grades your manufacturer permits. Most modern vehicles use one year-round multigrade oil.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
