Here is an artical which explains it all well. Better than i can anyway. :confused
Just a quick note on the fluid levels for anybody doing it themselves, although Ohlins specify 520 ml per leg the reality is unless you strip the forks there will always be residual oil in the fork (especially later models, external compression valve) that is trapped no matter how hard you try to pump it out. 520 ml is for a dry fork so always use the air gap to determine level. I got just under 500ml in mine to achieve the correct level on an oil change.
Give it a good pump with compression/rebound backed right off before measuring.
article -
THE SECRET WORLD OF FORK OIL
Go grab a basketball. Bounce it on the ground. See how quick it bounces back? That's rebound. Next, fill half of the basketball with water. Bounce it on the ground. See how sluggishly it returns? That's compression. The ball is a sphere to contain and restrict its contents. Depending on what you put in it, it will bounce high, bounce low or not bounce at all. Helium, air, water or lead will change the characteristics of a basketball.
Fork oil does the same thing to your bike's forks. Instead of helium, air, water or lead, forks use different viscosities of oil. MXA's guide to fork oil will help you understand what that means to you, your bike and any hoped-for win streaks.
QUESTION ONE: IS FORK OIL
AN OIL OR A FLUID?
Within the industry it's common to use both oil and fluid in the same sentence. That's because all oils are fluids. You will find that many of the best fork oils are labeled as a "cartridge fluid," but to our way of thinking that is more of a marketing move than actual fact. If you want to really amaze friends, you can also call fork oil "hydraulic fluid" or "hydraulic oil."
QUESTION TWO: WHAT IS HYDRAULIC DAMPING?
Hydraulics is a branch of science that deals with the practical applications of fluid in motion. Your motorcycle uses fluids in motion to resist movement. By transferring a quantity of oil from one end of the fork to the other through a series of small orifices or valves, the movement of the fork can be controlled. The more restrictive the valving or more viscous (heavier) the fluid, the slower the fork will move.
So, while the fork spring holds the front end up, it's the hydraulics that keep the fork from excessive bottoming or rebounding.
QUESTION THREE: WHAT DOES VISCOSITY MEAN?
Viscosity is the resistance of fluid to flow. It is measured by flowing a specific quantity of the fluid through a capillary tube (called a viscometer). The rate of flow is expressed in square centimeters per second, or more customary, in centistokes (cSt). The Society of Automotive Engineering uses the cSt measurement and converts it to a weight value (30 weight, 40 weight, 50 weight and so on). Those weights can be found on a can of motor oil. The SAE scale follows a very broad viscosity calibration scale.
QUESTION FOUR: IS FORK OIL MEASURED BY THE SAE SCALE?
No. Fork oil weights are derived from the industrial standards used for hydraulic applications, called the Saybolt Seconds Universal (SSU). This measurement uses a similar viscometer arrangement as used to determine a cSt value, but grades the oil using a much more sensitive viscosity calibration scale.
QUESTION FIVE: HOW DOES THE SAYBOLT SCALE WORK?
The thickness of hydraulic and fork oils are listed as the Saybolt Seconds Universal at 100 degrees C/viscosity index. Let's say that the numbers listed on the bottle read as 85/150. It means that the oil's SSU value at 100 degrees C is 85. Then, the flow of the oil is measured at 40 degrees C. The second number-150-is the value given to the difference in flow between the two temperatures. This is called the viscosity index (VI).
QUESTION SIX: WHAT'S THE IMPORTANCE OF VISCOSITY INDEX?
It tells how stable the fork oil remains from 104 degrees Fahrenheit up to 212 degrees Fahrenheit (or the boiling point of water). This is important because the higher the VI rating, the more stable the weight of the oil remains when it gets hot.
How does this apply to bike forks ? The friction created by sliding metal parts and oil flowing back and forth through valves creates heat. The more consistent the weight of the oil remains the less likely the fork's damping is to change as a moto progresses. If the oil gets hotter and thinner, the forks will get softer and faster.
Luckily, the oil in your fork seldom sees temperatures as high as 150 degrees Fahrenheit. The logic applied here is that a good fork oil should have a VI rating of at least 150.
QUESTION SEVEN: WHY DOES MY FORK OIL HAVE A WEIGHT?
Consumers like to shop for oil by weight. So, the motorcycle industry took the SSU/VI measurement and converted it to weights based on the same scale used by the SAE. But since the SAE weight schedule is so widely incremented, two cans of fork oil with different SSU viscosities of 80 and 100 can both be listed as a 5 weight (5wt). Yes, it is confusing!
QUESTION EIGHT: IS KAYABA 01 REALLY A 7-WEIGHT OIL?
This is where not having a finely calibrated oil weight rating scale confuses matters even more. Some mechanics claim that Kayaba 01-labeled a 5wt-is thicker than Showa SS-7 (and other 5wt cartridge fork fluids). They call it a "thick" 5wt or claim that it is really a 7wt. But, if you call Kayaba they will tell you that Showa's SS-7 is thicker than 01 oil.
Unfortunately, you have no choice but to let them call it what they want. The rule here is to find a reputable 5wt cartridge fluid that works best for you. When you find a fork oil that delivers the performance you want, stick with that brand. Just to clear the record, Showa SS-7, Kayaba 01 and Pro Circuit PC-01 are all produced by the same company in Japan. Except for color, they are the same weight oil.
QUESTION NINE: WHAT OIL
SHOULD MY FORK USE?
Every cartridge system production fork on the market comes standard with an oil rated as a 5wt. The factories use 5wt oil because it's thin enough to remain stable when heated past 100 degrees Fahrenheit and thick enough to lubricate the large surface areas sliding back and forth in a fork.
The starting point for every modern fork is 5wt oil. That is what your fork needs.
QUESTION TEN: HOW DO I FIND
THE BEST OIL?
Pro Circuit's suspension guru, Bones Bacon, recommends that a new bike first be ridden with the stock fork oil. Why? Modern suspension parts used hard coated internals and are filled with quality suspension fluid. While contamination from the factory and the risk of premature wear aren't an issue, breaking the bike in with the stock fork oil gives you a good feel of how the forks work stock. From this base setting you can judge all future service or mods.
If you're happy with the standard performance, have the fork serviced with the manufacturer's recommended replacement fork oil. If you like the way the suspension hop-up shop modified or serviced your fork using their recommended oil, stick with their brand of oil.
The Pro Circuit wrecking crew often switches fork oils instead of revalving a set of forks. Since a lighter weight oil offers less compression and rebound (and a heavier one vice versa), changing viscosities is identical to a revalve.
QUESTION 11: WHAT ABOUT 2-1/2 WEIGHT FORK OIL?
Most suspension fluids that are lighter than 5wt are designed specifically to be used in a shock. Fork oils heavier than 5wt get too thin when they get hot. Stick with 5wt fork oil.
QUESTION 12: WHY CAN'T I USE SHOCK OIL?
Fork and shock oil are two different animals. The oil inside the shock is completely air free, under 300 pounds of pressure and heated to 300 degrees Fahrenheit.
Fork oil is blended to lubricate and damp under less pressure and heat. Since the oil is contained in the same tube as the air, fork oil includes anti-foaming agents. Even if it wasn't a lighter oil, shock oil would immediately aerate and compromise the damping.
QUESTION 13: WHY DO HONDA AND SUZUKI CLAIM THEIR FORK OIL WORKS IN BOTH FORKS AND SHOCKS?
We think that it is an irresponsible and inadvertent marketing faux-pas. Pro Honda Suspension Fluid SS-7 5wt and 5wt Suzuki Fork and Suspension Fluid are manufactured by Showa and are designed to be used in a fork. Showa recommends SS-25 5wt for shocks. Pro Circuit's 02 is produced by Showa and is identical to SS-25. Kayaba's 5wt shock fluid is labeled K2C.
QUESTION 14: WHY CAN'T I JUST CHANGE THE OIL?
The best analogy is to think of worn fork internals just as you do of worn clutch plates. When the clutch starts slipping, a change of gearbox lube isn't going to keep the clutch from slipping for very long. It takes new plates, springs and, possibly, a clutch basket before clutch action returns to the same level of performance as when the bike was new. When hammered on the track, your forks oscillate wildly, heat up to over 100 degrees and put serious side loads on the shims and piston seals. All these parts need constant R&R.
