Developed for those who demand top performance from their suspension systems. Ideal for circle-track racing applications, cars, trucks, snowmobiles, ATVs, UTVs, and motorcycles. To cross-reference against your current product, visit
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Product data sheets
View, download, or save a copy of the Product Data Sheet or Safety Data Sheet for AMSOIL Shock Therapy® #5 Light Suspension Fluid.
AMSOIL Shock Therapy Suspension Fluid #5 Light is a fully synthetic shock and fork oil engineered for suspension systems that respond best to lighter damping and quicker rebounds. The #5 Light grade is one of two viscosities in the Shock Therapy lineup, paired with the heavier #10 Medium for systems that need stiffer damping. This grade fits sport, off-road, and powersports suspension hardware on motorcycles, dirt bikes, ATVs, UTVs, snowmobiles, cars, and trucks, including circle-track race vehicles. The lighter grade is also recommended for winter use, where cold-temperature flow is critical to consistent feel.
High viscosity index for fade-free damping
Aggressive riding and racing heat suspension fluid quickly. As the oil thins, damping fades, the chassis feels soft, and shock response becomes unpredictable. Shock Therapy #5 Light is built on synthetic base oils with a tested viscosity index of 209 by ASTM D2270, well into the range that resists viscosity loss across a broad temperature window. The result is consistent damping and rebound feel from the first lap or first jump through a long, hot session.
Wide temperature range for cold mornings and hot pavement
The #5 Light grade carries a tested pour point of -53 degrees C (-63 degrees F) by ASTM D97, so suspension internals stay fluid in deep winter conditions on snowmobiles, dirt bikes stored in unheated shops, and early-morning track sessions. At the high end, a tested flash point of 174 degrees C (345 degrees F) and fire point of 192 degrees C (378 degrees F) by ASTM D92 give the formula thermal headroom under sustained heavy use, including circle-track racing where shock bodies run hot.
Anti-wear protection for shock and fork internals
Every stroke loads pistons, shafts, bushings, and seal contact zones in a shock body. Shock Therapy #5 Light contains anti-wear chemistry that protects shocks and forks from premature wear and extends component life. In ASTM D4172 Four-Ball Wear testing run at 40 kgf, 75 degrees C, and 1,200 rpm for one hour, the formula posts a scar diameter of 0.44 millimeter, supporting longer service intervals between rebuilds.
Friction modifiers for smoother stroke action
Constant friction inside a shock body wastes energy and dulls suspension response. Shock Therapy #5 Light is formulated with friction modifiers that reduce energy loss across the stroke, so damping feedback stays linear and predictable. The same chemistry conditions seals to help reduce leaks and extend service life, which is especially useful in older shocks and cartridge forks where seal swell and shaft polish drift over time.
Anti-foam control for consistent suspension feel
Aeration is the enemy of shock performance. Air folded into the fluid compresses unpredictably, softening damping and giving the chassis a vague, mushy feel. Shock Therapy #5 Light is dosed with anti-foam agents that reduce aeration and foam, with tested results of 50/0, 40/0, and 50/0 milliliters across Sequences I, II, and III of ASTM D892. Quieter fluid means quieter, more predictable suspension at speed.
Race-grade chemistry built for tuning
The #5 Light grade lands at 4.4 cSt at 100 degrees C and 15.9 cSt at 40 degrees C by ASTM D445, at the lighter end of the common fork and shock oil range. Pair it with Shock Therapy #10 Medium when stiffer damping is needed, or use #5 Light on its own where the OEM or aftermarket builder calls for a lighter fluid.
Recommended applications and service intervals
Use AMSOIL Shock Therapy Suspension Fluid #5 Light in forks and shocks on snowmobiles, dirt bikes, motorcycles, ATVs, UTVs, cars, and trucks, including racing and recreational vehicles in extreme conditions. Agitate the fluid before use. The #5 Light grade is recommended for winter service and for systems where less damping and quicker rebounds are desired.
Use the #5 Light grade in suspension systems built by Bilstein, Custom Axis, Kayaba, Penske, Race Tech, Ryde FX, Showa, and WP that call for a lighter fluid.
Use the heavier Shock Therapy #10 Medium grade in suspension systems built by Fox, Koni, Ohlins, Race Tech, and WP that call for more damping and slower rebounds, and in Harley-Davidson forks that recommend type E or type B fork oil.
For an exact viscosity match against a current fork or shock oil, the AMSOIL shock fluid cross-reference lists side-by-side equivalents.
Service interval follows the suspension builder's published rebuild and oil change schedule. Heavy off-road, racing, and winter use shorten that interval.
For shock rebuilds, fill to the volume or air gap specified by the suspension manufacturer. Do not mix Shock Therapy #5 Light with shock fluids of a different viscosity grade unless the tuner is intentionally blending viscosities to fine-tune damping. To verify suspension hardware and viscosity choice for a specific machine, see the Synthetic Oil Distributor AMSOIL vehicle lookup guide and the AMSOIL dirt bike oil guide before ordering.
Order AMSOIL Shock Therapy Suspension Fluid #5 Light
AMSOIL Shock Therapy Suspension Fluid #5 Light ships in a quart bottle (STLQT-EA), case of twelve quarts (STLQT-CA), 5-gallon pail (STL05-EA), and 55-gallon drum (STL55-EA). The product is backed by the AMSOIL Limited Warranty. To match this fluid to a specific bike, sled, or shock build, start with the Synthetic Oil Distributor AMSOIL vehicle lookup guide, then review the dirt bike oil guide for related lubricants on the same machine. Order Now.
We're a shock manufacturer and live by this product, our customers love the ride when they go off-roading and compliment the suspension components. We would not be able to get those compliments without this shock oil, the performance and ability to stay consistent even in Arizona summers goes a long way. Amsoil also gets the products to us very quickly and with plenty of communication.
By Xavier from Phoenix, AZ on December 04, 2025.
AmsOil #5 Light Green Suspension Fluid
AmsOil always comes through for us, we order the 5 gallon pail of the #5 Light suspension fluid to complete the shock rebuilds from our suspension line. The customer all love the ride quality and consistency while tuning and performance. Multiple customers use these on their trail only vehicles and have definitely put the oil to the test going 60mph down canyon trails to completing multiple trails during Easter Jeep Safari and then driving home with no issues.
What viscosity is AMSOIL Shock Therapy #5 Light fork oil?
The #5 Light grade tests at 4.4 cSt at 100 degrees C and 15.9 cSt at 40 degrees C by ASTM D445, with a viscosity index of 209 by ASTM D2270. That places it at the lighter end of the common fork and shock oil range and pairs it with Shock Therapy #10 Medium for stiffer damping needs.
What is the difference between Shock Therapy #5 Light and #10 Medium?
The #5 Light grade is the lighter-viscosity fluid, suited to suspension systems where less damping and quicker rebounds are desired and to winter service. The #10 Medium grade is heavier and suited to systems that call for more damping control and slower rebounds. Many tuners run one grade exclusively, while some blend both to fine-tune feel.
Which fork and shock brands accept AMSOIL Shock Therapy #5 Light?
The #5 Light grade is recommended for suspension systems built by Bilstein, Custom Axis, Kayaba, Penske, Race Tech, Ryde FX, Showa, and WP that call for a lighter fluid. Suspension systems built by Fox, Koni, Ohlins, Race Tech, and WP that call for more damping and Harley-Davidson forks that recommend type E or type B fork oil should run Shock Therapy #10 Medium instead.
Can motor oil or ATF be used instead of fork oil?
No. Motor oil and automatic transmission fluid are formulated for engines and transmissions, not for the high-shear, low-volume environment inside a shock body or fork leg. They lack the viscosity index, anti-foam package, and seal-conditioning chemistry that suspension fluid needs. Use a purpose-built shock and fork oil such as Shock Therapy #5 Light or #10 Medium.
How often should fork and shock oil be changed?
Service interval follows the suspension manufacturer's published rebuild schedule. Aggressive off-road riding, dirt bike racing, frequent track days, snowmobile season, and winter operation shorten that interval. Inspect the fluid at every rebuild for darkening, contamination, or smell, and replace whenever it looks or smells off.
Is Shock Therapy #5 Light a winter shock oil?
Yes. The #5 Light grade is recommended for winter use, with a tested pour point of -53 degrees C (-63 degrees F) by ASTM D97. Cold-temperature flow stays consistent in snowmobile shocks and in dirt bike or street bike forks stored in unheated garages.
Does AMSOIL Shock Therapy come with a warranty?
AMSOIL Shock Therapy Suspension Fluid is backed by the AMSOIL Limited Warranty. Suspension components themselves remain covered by the manufacturer that built or rebuilt them, on the maintenance interval that builder specifies.
