This comes from
http://www.strappe.com/suspension.html
There is more info there so take a look. knowing what the reason for the air is helps. So now you know it's not about haw much oil is in each fork but how much air is each fork.
Oil level:
The fork oil's primary function is to be pushed through small holes to damp the rebound of the spring. In an automotive shock, this is the only function of the oil.
However, in a bike fork there is the spring, the oil, and the trapped air above the oil.
This air is important - air, when compressed, acts like a spring, but with a rising rate. That is, the more it's compressed the more force it takes to compress it a bit more. Most fork springs, on the other hand, are linear (straight rate).
The idea is to set the oil level so that the right amount of air is trapped to keep the forks from solid metal contact at full compression. (Obviously, bottoming the fork internals leads to a harsh ride and component damage.) The right amount of oil will allow nearly all of the fork travel to be used in riding and stopping.
How do you find out what's needed?
First, with the cap and spring out of the fork, find out what the maximum travel is (when the fork internals bottom out) {Ihave not done this but the paso book said 140mm}. Then reassemble the forks and install the recommended amount of oil.
Second, once the forks are on the bike, take a zip tie (plastic cable tie) and put it around the fork tube above the slider - make it tight enough that it'll stay where it's pushed by the slider's dust cap.
Third, go for a ride. Do some hard stops with the front brake. Do stoppies if you can.
Fourth, back in the shop, compare the position of the zip tie with the previously measured maximum travel. I like to get the working travel to within 1/4 inch of maximum.
If the working travel is too little, remove a bit of oil.
If it's near or equal to the max, add some.
The best way to do this is to remove the fork springs (support the front end first) and measure the oil level in each leg with the forks totally compressed.
Measure the distance from the top of the tube to the oil level. If you want to make changes, I'd suggest a level change of no more than 10mm at a time.
It's hopeless to try to measure oil volume accurately once it's in the forks, that's why we measure level instead.
Here's an example:
Take a look at my SV650's log - you'll see in column 7 I set the oil level at 124mm, which yielded a total travel of 112mm, 10mm short of the target of 122mm. In column 8 I reduced it to 127mm, for a total travel of 115mm. In column 9, I dropped it further to 132mm, which produced travel of 122mm. Right on target.
Then in column 10 I cut the springs to make them stiffer yet and started over.
Oh, yeah: you should leave the zip tie on the fork leg. It will tell you if your travel changes, which will happen if you lose any fork oil... and anyway, it lets the cognoscenti know you're hip! Almost as good as tiny rubber balls on the edges of your tires.