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    Suspension Fluid

    AMSOIL Shock Therapy Suspension Fluid — Synthetic Fork & Shock Oil

    AMSOIL Shock Therapy Suspension Fluid is the best synthetic shock and fork oil for snowmobiles, dirt bikes, ATV/UTV, mountain bikes, and off-road trucks. Available in #5 Light and #10 Medium, it holds viscosity across wide temperatures, resists fade through repeated compression cycles, and extends fork and shock seal life.

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    Why Run AMSOIL Shock Therapy Suspension Fluid?

    • Two viscosity grades — #5 Light & #10 Medium
    • Synthetic base — consistent viscosity from cold start to full heat
    • Resists fade & viscosity loss under repeated compression cycles
    • Anti-foam additives prevent cavitation inside the damper
    • Seal conditioners extend fork and shock seal life
    • Anti-wear protection for piston, shaft & valving
    • Engineered for snowmobile, dirt-bike, ATV/UTV & off-road truck shocks
    • Compatible with most other fork and shock oils

    Product Lineup

    Shop AMSOIL Shock Therapy Suspension Fluid

    Synthetic suspension fluid in three viscosity weights — engineered for forks, shocks, monotube and twin-tube dampers, and coilovers across powersports, off-road, and performance applications.

    Shop AMSOIL Shock Therapy® #5 Light Suspension Fluid
    AMSOIL Shock Therapy® #5 Light Suspension Fluid

    AMSOIL Shock Therapy® #5 Light Suspension Fluid

    Light-viscosity synthetic suspension fluid for forks and shocks calling for a 5-weight oil. Ideal for mountain bikes, dirt-bike forks, lighter snowmobile shocks, and applications where small-bump compliance and fast rebound matter most.

    Fitment:

    • • Mountain bikes, dirt-bike forks, lighter snowmobile shocks
    • • Applications specifying 5-wt fork/shock oil
    • • Cold-climate operation

    Specs:

    • • Light viscosity grade (~5 wt)
    • • High viscosity index — prevents fade under extreme temperatures
    • • Consistent, smooth damping
    • • Seal conditioners for extended service life
    • • Race-grade technology

    Product Code: STL

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    Shop AMSOIL Shock Therapy® #10 Medium Suspension Fluid
    AMSOIL Shock Therapy® #10 Medium Suspension Fluid

    AMSOIL Shock Therapy® #10 Medium Suspension Fluid

    Medium-viscosity synthetic suspension fluid — the most-used grade across powersports. Engineered for snowmobile coilovers, dirt-bike shocks, ATV/UTV monotube and twin-tube dampers, and most performance applications.

    Fitment:

    • • Snowmobile coilovers, dirt-bike & ATV/UTV shocks
    • • Most powersports applications (default grade)
    • • Performance street suspension dampers

    Specs:

    • • Medium viscosity grade (~10 wt)
    • • High viscosity index — prevents fade under extreme temperatures
    • • Consistent, smooth damping
    • • Seal conditioners for extended service life
    • • Race-grade technology

    Product Code: STM

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    Weight Comparison

    Light vs. Medium Suspension Fluid — Which Weight Do I Need?

    Always start with the OEM or shock-tuner spec. As a guide: lighter weights feel more plush with faster rebound; heavier weights add compression damping and slow rebound for high-load and high-speed work.

    WeightDamping FeelTypical ApplicationAMSOIL Product
    Light (#5)Plush, fast rebound, supple small-bump complianceMountain bikes, dirt-bike forks, lighter sled shocks, cold climatesShock Therapy #5 Light (STL)
    Medium (#10)Balanced compression & rebound — the default for most powersportsSnowmobile coilovers, dirt-bike shocks, ATV/UTV monotube & twin-tube dampers, performance streetShock Therapy #10 Medium (STM)

    Who Uses Shock Therapy Suspension Fluid?

    Anywhere a fork or shock damps a moving wheel, AMSOIL Shock Therapy delivers more consistent feel and longer service life than conventional shock oil:

    Snowmobiles

    Coilover shocks on Ski-Doo, Polaris, Arctic Cat, Yamaha sleds. #10 Medium is the workhorse; #5 in deep cold.

    Dirt Bikes

    Front fork and rear shock service. #5 Light typical for forks, #10 Medium for shocks — confirm OEM spec.

    ATVs & UTVs

    Monotube and twin-tube dampers on sport quads and UTVs. #10 Medium covers most powersports applications.

    Mountain Bikes

    Suspension forks and rear shocks. #5 Light is the common service weight.

    Off-Road Trucks

    Coilover shocks on prerunners, trophy trucks, and overland rigs. #10 Medium with revised valving for high load and speed.

    Performance Street

    Coilover dampers on track and time-attack cars. Match the damper manufacturer's recommended weight.

    The Complete Guide to AMSOIL Shock Therapy Suspension Fluid

    What suspension fluid actually does

    Inside every fork and shock, a piston with calibrated valving forces oil through small ports as the suspension compresses and rebounds. That hydraulic resistance is your damping. The oil also lubricates the shaft and bushings, transfers heat away from the piston, and keeps seals supple. As the oil heats, shears, or foams, damping changes — that's why fluid quality directly affects feel, consistency, and shock life.

    Why synthetic — viscosity stability is everything

    A great shock oil holds its viscosity across a huge temperature window. Cold-start at 20°F it should still flow well enough that the first few hits aren't harsh; running hot at 200°F+ after a long moto, it should still be thick enough to maintain damping. AMSOIL Shock Therapy uses synthetic base stocks specifically chosen for high viscosity index — meaning less viscosity drop as the damper heats up — so the suspension feels the same on lap 20 as it did on lap 1.

    Foam and cavitation kill consistency

    When a shock cycles fast, the oil can foam or cavitate around the piston — vapor compresses, oil doesn't, and the result is sudden loss of damping (fade). AMSOIL Shock Therapy includes anti-foam additives that suppress bubble formation, plus the synthetic base resists foaming better than conventional shock oil. The practical benefit: more consistent damping through repeated compression cycles in whoops, rock gardens, and washboards.

    Seals last longer with the right oil

    Fork seals and shock seals fail when they dry out, swell from incompatible chemistry, or wear from abrasive contamination. AMSOIL Shock Therapy includes seal conditioners that keep elastomers pliable and properly sized. That translates to fewer leaks, less stiction, and longer service intervals between rebuilds.

    Tuning weight — light, medium, heavy

    Going one step lighter (e.g. #10 to #5) opens up the damping — more plush small-bump compliance and faster rebound. For heavier damping, AMSOIL no longer offers a #20 Heavy grade — instead, work with your shock tuner on revised valving or shim stacks. Tune in single steps, ride or drive a known section to evaluate, and stay within the shock manufacturer's recommended weight range.

    Suspension Fluid — Frequently Asked Questions

    What is the best synthetic suspension fluid for shocks?▾
    AMSOIL Shock Therapy Suspension Fluid is widely regarded as the best synthetic shock and fork oil for powersports and off-road use. Available in #5 Light (STL) and #10 Medium (STM), it holds viscosity across wide temperature swings, resists fade through repeated compression cycles, and extends fork and shock seal life vs. conventional shock oil.
    What is Shock Therapy suspension fluid used for?▾
    AMSOIL Shock Therapy Suspension Fluid is engineered for the dampers, coilovers, and forks on snowmobiles, dirt bikes, mountain bikes, ATVs, UTVs, off-road trucks, and performance street applications. It controls compression and rebound damping while resisting foam, fade, and viscosity loss under hard, repeated cycling.
    Is synthetic shock fluid better than conventional?▾
    Yes. Synthetic suspension fluid holds a more consistent viscosity across cold-start to full-heat operation, resists shear-induced viscosity loss under repeated cycling, and includes seal conditioners and anti-foam additives that conventional shock oil typically lacks. The result is more consistent damping feel and longer fork and shock seal life.
    What viscosity suspension fluid do I need?▾
    Always check your shock or fork OEM/tuner spec first. As a starting point: #5 Light (STL) for mountain bikes, dirt-bike forks, and lighter sled shocks; #10 Medium (STM) for most powersports including snowmobile coilovers, dirt-bike shocks, and ATV/UTV dampers. Note: AMSOIL no longer offers a #20 Heavy grade — for heavier damping use #10 Medium with revised valving or consult your shock tuner.
    Can AMSOIL suspension fluid be used in forks and shocks?▾
    Yes — AMSOIL Shock Therapy Suspension Fluid is engineered for both forks and shocks (monotube, twin-tube, and coilover dampers). It is not a brake fluid, gear oil, or steering fluid; only use it where the OEM or suspension tuner specifies fork oil or shock oil of the corresponding viscosity weight.
    How often should I change suspension fluid?▾
    Follow your OEM or shock manufacturer interval — typically once per season for snowmobiles, every 25–50 hours for dirt-bike forks, and per service interval for ATV/UTV dampers. Race and desert teams often refresh fluid after every event. Dirty, dark, or foamy fluid should be changed immediately.
    Will heavier suspension fluid stiffen my shocks?▾
    Effectively, yes — heavier viscosity increases compression and rebound damping, making the suspension feel firmer and more controlled at speed. Going lighter does the opposite (more plush, faster rebound). Tune one weight at a time and confirm against the shock manufacturer's recommended range before going outside it.

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