Motorcycle suspension, as hard as it may be to believe, profit was one of the primary motivations in the design of you motorcycle's suspension. The motorcycle manufacturers don't want your motorcycle to have the best suspension. They want it to have the best suspension they can afford. When saving a few thousand Baht filters down to shock and fork hydraulics, it means less-than-optimal suspension fluid and over-engineered components. But of your motorcycle has adequate suspension, it will perform so much better.
When some motorcycle shops and mechanics can improve your suspension performance partly through reducing stiction. Making parts move easier means using thinner piston bands and fewer sealing O-rings. Modified suspension does more work with less, and thus requires more frequent service.
The increased load on modified suspension seals and bushings is mostly offset through the use of higher grade suspension fluid. When professionals modify your suspension, they use usually the highest quality components and fluids available.
The biggest difference between stock suspension and aftermarket suspension fluid is stability. Performance fluid will have the best viscosity index, and the thickness of the oil, as well as fork and shock damping, will remain more consistent all the time.
Most suspension fluid is petroleum-based, as is most stock fork and shock oil, but the manufacturer used standard-base petroleum. The suspension fluids that suspension modify shops use are always a step or two better. Aftermarket fluid includes the best polymers to keep the oil viscosity stable when hot. It also has the best additive system, including anti-wear, anti-foam, extreme-pressure, anti-stiction and anti-corrosion modifiers.
The biggest misconception people have is that changing the fluid in a worn shock or fork is enough to fix its flaws. It isn't. Think of you suspension components as though they were a burnt clutch. New oil won't make a burnt clutch work. The same can be true of the internals of your forks or shocks.
Valving shims lose their snap, or resistance to bending, is what controls the ebb and flow of compression and rebound damping. When the shims wear out, the suspension boings.
How do you know it the shims have worn out? Actually discovering the wear is difficult, which is why you need to be a professional to discover losses that are measured in thousandths of an centimeter. New seals, bushings, O-rings and shims are required to bring old suspension back to specifications.
Fork suspension oil viscosity is not always the same. Regular forks operate around 43c degree and use 5w suspension oil. Shocks operate at 148c degree and use a lighter 3w oil.
You suspension maintenance schedule depends on how you ride. After 6 months, the suspension components of a modest rider might look brand new. But, an aggressive rider will wear the shock and forks out in less than 6 months. Additionally more, dirt, more bumps break down shims and fluid quicker than track-days.