Basically all you need to know about your fork oil or shock oil is that you must use the right viscosity (thickness of oil). In all modern dirt bikes - MX and Enduro – use 5 weight oil for your forks and 2.5 weight oil for your shock. Besides it is best to use the oil manufacturer that is indicated in your manual. Use preferably the top of the range fully synthetic oil. Really that is all you need to know. If you want to know why, then read on.
Changing viscosity does have a significant effect on damping. It will change both rebound and compression damping at the same time.
To then find the right balance can only be done by a suspension specialist who can re-valve the suspension. Changing the internals of the suspension is a very delicate mission. This should not be done by anybody who has not some sort of specialized training on suspension. In the "shims" article or in the "suspension specialist myth busting" article you can read on why this task needs to be done by professionals.
Why does it then even cross people’s minds to change the viscosity of the oil? Older model forks used simple valves with holes. The only way to change damping then was by changing oil viscosity or drilling and re-welding the holes. Modern forks (all MX and Enduro bikes today) use shim stacks with complex valves. They are designed to be used with 5 weight fork oil and 2.5 weight shock oil.
Thicker oils will have negative effects on the suspension. In general thicker oils will make the damping (both rebound and compression) much harder. Until a point when by being too thick the oil cannot pass the valve fast enough. If this happens you get air build up behind the piston and momentary total loss of damping which is called cavitation. This is very undesirable and happens fairly easily with thicker oils. That is the main reason why thicker oils must be avoided.
Quality suspension oils have a high viscosity index. That means the oil
maintains its thickness well under temperature changes. Forks remain
fairly cool during function but shocks reach temperatures well over 100
degrees Celsius. Therefore it is important that the oil viscosity does
not change too much. For good performance it is also necessary to stick to the recommended service intervals.
If you use poor quality oil with a low viscosity index in shocks then you will experience extreme fading and lack of performance. The oil will thin too much and thus not offering the desired damping.
Now if you think you can use poor quality oil in your forks because of
the lower temperatures you are wrong. Fork oils concentrate more on
lubrication than the viscosity index. Lubrication is vital for good fork
performance. So therefore the oil must have good anti foaming agents,
good lubrication properties and must not harden seals or cause them to
swell too much.
Most leading manufacturers of quality suspension oils will meet the above requirements. It is always best though to use the oils specifically designed for the fork or shock in question. Oils mentioned in your manual are always the safest. Then you know they will be compatible with your suspension and its materials.
Remember MX and Enduro bikes operate under extreme conditions. That is why you should use only the best oils to cope with the high demands you put on your suspension.
Make sure that you maintain your fork seals regularly. You don't need a specialist for that. And you can even do it at the service points during the race if it is peak performance you are after.
The Dirt Bike Garage Manual
HOW TO GUIDE for "do it yourself" riders