The second number (150) denotes the difference in flow between the two temps, aka the viscosity index (VI). The flow of the oil is then measured at 40º C. For example, if the numbers listed on the bottle read as 75/150, that means the oil's SSU at 100º C is 75. The thickness of hydraulic and fork oils are listed as the Saybolt Seconds Universal at 100 degrees C/viscosity index. The SSU uses a similar measurement standard to the one used to determine a cSt value, but oil grades are based on a more sensitive scale of viscosity calibration. Instead, they rely on the Saybolt Seconds Universal (SSU), a standard used for hydraulic applications. In addition, fork oil weights aren't measured using the SAE scale.
However on motor oil which is listed like 5w30, '5w' stands for '5 winter' so this is where some of the confusion arises. The Society of Automotive Engineering (SAE) uses cSt measurements and converts it to a weight value like 5wt, 10wt, which may sound familiar because that's what you're used to seeing on a container of motor oil. Viscosity is the resistance of fluid to flow, measured in square centimeters per second, aka centistokes (cSt). Let's review a few terms before we go further. The reasons behind this are beyond the scope of this article, but the basic gist is that the SAE weight system is inconsistent due to the way the SSU/VI systems are measured. If you take nothing away from the tech stuff below, it should be that if you're trying to pick what suspension oil to use, don't go by ratings that use 'wt' - it's not going to be that useful at least as far as comparisons across brands. One brand's 5wt is not the same as another brand's 5wt, and anyway 'wt' or 'weight' isn't even an accurate way of measuring the performance of a suspension oil.
Suspension oil weights are not standardized Let's cover a few things about suspension oil that - if you're like me - you probably didn't know before now. We all know that taking care of our suspension is essential for a smooth-running bike that performs well, but in doing so, we should try to make choices about the products we use that are better for ourselves and the planet. What's also lurking inside, though, are petroleum-based oils that are harmful to mechanics when not handled carefully, and harmful to the environment if not disposed of properly. We can't see what's in our forks and shocks, which makes their inner workings a beautiful mystery of shims, valves, and pistons that help fuel the fun on two wheels.
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