A viscosity grade is not a quality score. A 10W-50 is not automatically more protective than a 10W-40, and a 5W-40 is not automatically too thin. The grade describes how the oil falls within standardized viscosity limits at low and operating temperatures.
Read the two numbers separately
The number before the W describes low-temperature cranking and pumping behavior. A lower number can support oil circulation during colder starts. The second number describes the oil's high-temperature viscosity grade. It does not mean the oil becomes thicker as it heats; every engine oil thins with heat, but the higher grade remains within a higher viscosity range at the test temperature.
| Grade | Typical reason it appears in a motorcycle manual |
|---|---|
| 10W-40 | A common balance for sport and street motorcycles across a broad ambient range. |
| 10W-50 | Specified by some engines or temperature charts where a higher operating-temperature grade is desired. |
| 15W-50 | Often listed for warmer conditions or particular engine designs, with less cold-start range than a 10W grade. |
| 20W-50 | Common in some air-cooled, V-twin, or older applications when explicitly approved. |
Start with the manual's viscosity chart
Many manuals allow more than one grade based on ambient temperature. Stay inside that chart unless the engine builder provides a documented alternative. The clearances, oil pump, bearing loads, gearbox, clutch, and cooling system were considered when the manufacturer published its recommendation.
Track use does not automatically mean “go thicker”
Track use raises load and often oil temperature, but changing viscosity can affect flow, drag, pressure, shift feel, and clutch behavior. Before changing grades, verify actual temperature, oil level, cooling-system condition, and the manufacturer's permitted range. A high-quality 10W-40 at the proper level may be preferable to an unapproved 10W-50 chosen by habit.
Specifications are separate from viscosity
Two bottles marked 10W-40 can have different additive systems and approvals. For many wet-clutch motorcycles, JASO MA or MA2 is material. Modern motorcycles may also specify an API service category. Match viscosity and the required performance standard.
Common mistakes
- Choosing a grade from another model with a different engine.
- Using automotive “energy conserving” oil without confirming wet-clutch suitability.
- Changing grade to mask fuel dilution, overheating, low oil level, or engine wear.
- Reading pressure alone as proof of lubrication quality.
- Assuming the most expensive oil permits an extended interval.
A practical selection process
- Find the exact model-year owner's manual.
- Record the allowed viscosity range and required JASO/API specifications.
- Consider the coldest start and hottest sustained operation.
- Choose a product that clearly publishes the required standards.
- For a modified race engine, follow the builder's written recommendation.
- Log consumption, shift quality, temperature, and change interval rather than relying on color.
Bottom line: use 10W-40 when it is the manual's default and your temperature range supports it. Use 10W-50 or 15W-50 when the manual, its temperature chart, or a qualified engine builder supports the change—not simply because the bike will see a track.
Related product
MOTUL 7100 4T Synthetic Oil — 10W-40 — 1L
A current API SP/JASO MA2 option for motorcycles specifying 10W-40.
Related product
MOTUL 7100 4T Synthetic Oil — 10W-50 — 1L
A higher hot-viscosity option only for applications whose manual or builder permits 10W-50.
Sources and further reading
Specifications, prices, approvals, and track rules can change. Checked July 2026.