According to the American Voyager Association:

"We would suggest changing the fork oil should be a regularly scheduled maintenance item. It should be done every other year! Recommended oil is 10w fork oil. There are two ways to go about changing the oil. The easy way is to drain out the old oil from the plug at the bottom of the fork. Remove the wheel and pump the lower fork tube up and down until no more oil comes out. When changing the oil, the shop manual says replace it with 330mL of new oil. However, if you completely remove the fork, disassemble it, and clean it, the shop manual recommends 391mL + or- 2.5mL. I have never figured out why, but many times each fork will require more or less fluid to reach the correct height in the fork tube. The correct way to put just the right amount of fluid in, is to pump the lower fork tube up and down several times to get the fluid into all of the cavities. After doing this, fully compress the fork tube and measure down from the top 140mm + or - 2mm. This is the correct amount of oil to have in each fork with stock fork springs. Sometimes you have to add some fluid, other time you need to take some out. But this is the only way to make sure both forks have exactly the right amount of fluid.