To change the fork oil the best way, you take the forks off and hold them upside down to drain the oil, and some rinse them out with kerosene. My quicker method not quite as good is to buy a long piece of clear plastic tubing, wrap some wire around the first foot to make it stiffer and go into the forks easier, and put it down into the forks after removing the springs, then use mouth suction to get the old fork oil out. You can get most of the old oil out this way, and you can add more new oil to dilute what oil is left to get more of the old oil out. The clear hose keeps you from sucking the oil into your mouth. My repair manual has both a distance from the top and a volume measurement for the fork oil- with this method you have to go by distance. I found adding a little more fork oil than stock makes the forks stiffer, and after a few thousand miles the extra oil has not blown out the fork seals.