AMSOIL 10W-40 100% Synthetic Dirt Bike Oil
AMSOIL 10W-40 100% Synthetic Dirt Bike Oil is a JASO MA, API SG/SJ/SH/SL four-stroke motorcycle oil engineered for hard-launched motocross, enduro, and trail bikes that demand consistent clutch engagement and high-rpm wear protection. The 10W-40 grade fits dirt bikes whose service manual calls for SAE 10W-40 motorcycle oil and is recommended for separate-sump applications, including the Honda CRF, and many two-stroke dirt bike transmissions.
Consistent clutch feel under race-start abuse
Friction-durability is the headline property for any wet-clutch motorcycle oil, and AMSOIL Synthetic Dirt Bike Oil is built around it. The formula helps minimize clutch slippage, fade, and chatter so the lever stays predictable from the first gate drop through long motos. In an extreme simulated-start test, AMSOIL Synthetic Dirt Bike Oil continued to deliver consistent clutch feel after 32 simulated race starts, while a leading original equipment manufacturer (OEM) branded oil showed inconsistent clutch-lever action and poor clutch feel after 16 starts. Clutch plates run on the AMSOIL oil remained clean and in good overall condition, while plates run on the OEM oil showed discoloration and wear.
Wear protection for gears, bearings, and pistons
Four-stroke dirt bike engines spin past 12,000 rpm and share the engine sump with hardened transmission gears that chop conventional oils thin. The synthetic base stocks in AMSOIL 10W-40 Synthetic Dirt Bike Oil naturally resist mechanical shear and viscosity loss, keeping a thicker protective film between gear teeth and bearing surfaces under high-rpm operation and extreme heat. The premium anti-wear additive package provides an extra measure of protection against gear, bearing, and piston wear, helping extend clutch life and reduce mid-season top-end maintenance.
Shear-stable viscosity for high-rpm operation
AMSOIL 10W-40 Synthetic Dirt Bike Oil delivers a kinematic viscosity of 14 cSt at 100°C (ASTM D445) and a viscosity index of 167 (ASTM D2270), which together describe a fluid that stays in its target SAE 10W-40 range across the temperature swings of summer trail riding and competition use. High-temperature, high-shear (HTHS) viscosity measures 4.0 cP at 150°C (ASTM D5481), the load-bearing film thickness range expected of a premium 10W-40 motorcycle oil that has to lubricate gears in addition to engine internals.
Heat tolerance, low volatility, and cold flow
NOACK volatility (ASTM D5800) measures 4.8 percent weight loss, which means very little of the oil flashes off in the hottest parts of the engine, helping control oil consumption between changes and reducing deposit formation on pistons and ring lands. The flash point is 240°C (464°F) and the fire point is 270°C (518°F) per ASTM D92, both well above the operating temperatures of an air-cooled or liquid-cooled four-stroke dirt bike top end. The pour point reaches -40°C (-40°F) per ASTM D97, supporting confident cold cranking on cool mornings before practice.
JASO MA wet-clutch spec and API service categories
AMSOIL 10W-40 100% Synthetic Dirt Bike Oil meets JASO MA, the wet-clutch friction standard developed for shared-sump four-stroke motorcycle engines, and carries API SG, SJ, SH, and SL service categories. Riders running modern Honda CRF, Yamaha YZ-F, Kawasaki KX-F, Suzuki RM-Z, and Husqvarna four-stroke models whose owner manuals call for a JASO MA, SAE 10W-40 motorcycle oil can use this product as a direct factory-fill replacement. For machines that require a heavier grade such as 10W-50 or 10W-60, the AMSOIL Synthetic Dirt Bike Oil family also lists those viscosities.
Recommended applications and service intervals
Use AMSOIL 10W-40 100% Synthetic Dirt Bike Oil in any dirt bike whose service manual calls for SAE 10W-40 motorcycle oil, including four-stroke models made by Honda, Yamaha, Kawasaki, Suzuki, and Husqvarna. The 10W-40 grade is recommended for separate-sump applications, including the Honda CRF, and for many two-stroke dirt bike transmissions where a JASO MA, 10W-40 grade is specified. Riders whose machines call for 10W-50 or 10W-60 should select those grades from the AMSOIL Synthetic Dirt Bike Oil family rather than substituting this 10W-40.
- Change AMSOIL 10W-40 Synthetic Dirt Bike Oil according to the vehicle manufacturer's recommendations; racing schedules typically tighten this interval based on hours and conditions.
- Compatible with other synthetic and conventional motor oils; mixing shortens life expectancy and reduces performance benefits, and extended intervals are not supported on mixed fills.
- Aftermarket oil additives are not recommended for use with AMSOIL Synthetic Dirt Bike Oil.
- Confirm viscosity grade, JASO classification, and capacity for a specific model using the AMSOIL vehicle lookup guide.
Pair AMSOIL 10W-40 Synthetic Dirt Bike Oil with a fresh, OEM-spec motorcycle oil filter at every change, replace the crush washer on the drain plug, and follow the manufacturer's torque values to prevent leaks.
Typical technical properties
| Property | Value (DB40) |
|---|
| Kinematic Viscosity at 100°C, cSt (ASTM D445) | 14 |
| Kinematic Viscosity at 40°C, cSt (ASTM D445) | 86.4 |
| Viscosity Index (ASTM D2270) | 167 |
| Flash Point, °C (°F) (ASTM D92) | 240 (464) |
| Fire Point, °C (°F) (ASTM D92) | 270 (518) |
| Pour Point, °C (°F) (ASTM D97) | -40 (-40) |
| NOACK Volatility, % weight loss (ASTM D5800) | 4.8 |
| High-Temperature/High-Shear Viscosity at 150°C, cP (ASTM D5481) | 4.0 |
| Four-Ball Wear Test, scar diameter, mm (40 kgf, 150°C, 1800 rpm, 1 hr) | 0.57 |
| Foam Test (ASTM D5182, Sequences I, II, III) | 0/0, 20/0, 0/0 |
| Total Base Number | 7.6 |
Order AMSOIL 10W-40 100% Synthetic Dirt Bike Oil
AMSOIL 10W-40 100% Synthetic Dirt Bike Oil ships in a quart bottle (product code DB40QT-EA) and a case of twelve quarts (product code DB40QT-CA). The product is Warranty Secure, keeps the factory warranty intact when used as a high-performance replacement for vehicle manufacturer branded products, and is backed by the AMSOIL Limited Warranty. For application match by year, make, and model, start with the AMSOIL vehicle lookup guide, and to compare 10W-40, 10W-50, and 10W-60 dirt bike grades plus 2-stroke premix options, see the AMSOIL dirt bike oil guide. Order AMSOIL 10W-40 100% Synthetic Dirt Bike Oil.
Customer reviews
Great wear protection!
I use this in all my dirt bikes. It quiets down every engine I put it in. Never had a clutch slip. I do oil changes ever 4-6hrs of moderate track riding. 2011 crf450r, '16 crf250r, '23 kx65, '22 crf50
By Josh on June 06, 2026.
Would recommend this to a friend.
Smooth as Silk
I was running MAXIMA 10-40w 4 stroke...With this 06 yz450 has a Problem from shifting from Third to Fourth when RPM\`s are high...Its common with 06,07.....But did some researching...Had a Motocross Racer suggest running AMSOIL 10w-40 weight for better low friction from Third to Fourth.....So far Hauling Ass....Thanks # 007
By William from North Carolina on May 24, 2026.
Would recommend this to a friend.
Atv
Fantastic oil my Honda ATV ran so smoothly after using this oil.
By Paul from Caledon Canada on May 18, 2026.
Would recommend this to a friend.
Read all 187 reviews
Frequently asked questions
Is AMSOIL 10W-40 Synthetic Dirt Bike Oil JASO MA approved for wet clutches?
Yes. AMSOIL 10W-40 100% Synthetic Dirt Bike Oil meets JASO MA, the friction standard developed for shared-sump four-stroke motorcycle engines with wet clutches, and it carries API SG, SJ, SH, and SL service categories. The clutch-friction performance has been validated in an extreme simulated-start test against a leading OEM-branded oil.
How often should AMSOIL 10W-40 Synthetic Dirt Bike Oil be changed?
Change AMSOIL 10W-40 Synthetic Dirt Bike Oil according to the dirt bike manufacturer's recommendations in the owner manual. Race teams and hard-charging riders typically tighten that interval based on engine hours, race weekends, fuel quality, and dust exposure. For application-specific guidance and capacity, use the AMSOIL vehicle lookup guide.
Can 10W-40 car motor oil be substituted in a dirt bike?
Substituting passenger-car 10W-40 in a dirt bike that calls for a motorcycle oil is not recommended. Passenger-car oils often contain friction modifiers that can cause wet-clutch slippage and are not formulated for the shear loads of motorcycle transmission gears. AMSOIL 10W-40 Synthetic Dirt Bike Oil is JASO MA wet-clutch friendly and shear-stable for shared-sump and separate-sump dirt bike applications.
What dirt bikes is the 10W-40 grade recommended for?
The 10W-40 grade is recommended for dirt bikes whose service manual calls for SAE 10W-40 motorcycle oil, including four-stroke models from Honda, Yamaha, Kawasaki, Suzuki, and Husqvarna, and for many two-stroke dirt bike transmissions. It is specifically recommended for separate-sump applications such as the Honda CRF. Machines that specify 10W-50 or 10W-60 should use the matching grade from the AMSOIL Synthetic Dirt Bike Oil family.
Will AMSOIL Synthetic Dirt Bike Oil void a factory warranty?
No. AMSOIL 10W-40 Synthetic Dirt Bike Oil is Warranty Secure, keeping a factory warranty intact when used as a high-performance replacement for vehicle manufacturer branded oils, and is backed by the AMSOIL Limited Warranty. To confirm the right grade for a specific dirt bike, consult the AMSOIL dirt bike oil guide and the owner manual.
Can AMSOIL 10W-40 Synthetic Dirt Bike Oil be mixed with conventional oil?
AMSOIL 10W-40 Synthetic Dirt Bike Oil is compatible with other synthetic and conventional motor oils, so a mid-trip top-off will not cause damage. Mixing oils, however, shortens the life expectancy of the AMSOIL formula and reduces its performance benefits, and extended drain intervals are not supported on mixed fills. Aftermarket oil additives are not recommended.
