Good motorcycle fork oil has some basic requirements: viscosity must be suitable, high viscosity index, good anti foaming agents and good lubrication properties. Most leading oil manufacturers of quality suspension oil will meet those requirements.
But the best fork oil won’t help you when you ride with the wrong oil
level. And believe it or not there are some crazy oaks out there who
just change the oil level without really knowing what the effect will
be. Almighty crashes can be the result.
Remember that the relationship between oil level and progression is not linear but exponential. In other words: adding 20ml more oil may not make a huge difference the first time. Add another 20ml and you may suddenly find the fork becomes very hard deeper in the stroke.
Double the amount of oil does not double the air pressure relationship. It increases it to an exponent of around 2.6 times. Your fork becomes harder and harder as it compresses the more oil you add. But the initial fork stroke remains similar. If you add too much oil your fork will become very uncomfortable and hard as it compresses.
The graph shows the effect of adding oil. The initial part of the fork stroke is not affected too much in both examples.
As pressure builds up in the fork with the smaller air volume (higher
oil level) it gets much more progressive than the fork with the larger
air volume (lower oil level).
So feel free to experiment with oil volume, but stick within the maximum and minimum levels in your manual. If you experiment make 10ml changes at a time in order to feel the effect. Be warned you may find your fork suddenly becomes way too hard if you add a little too much fork oil.
The safest and probably best thing to do is to use standard oil level. This has been carefully worked out to match the damping and springs in your fork. No, not all engineers in the motorcycle industry are ninkumpupses. They actually know what they are doing!
The Dirt Bike Garage Manual
HOW TO GUIDE for "do it yourself" riders