You may have heard it around the pits or read about it in a magazine. The questions is how exactly does adding or removing oil from your forks affect the suspension?
“Adding oil to your forks reduces the air volume. Since air is compressible, a smaller airspace is harder to compress than a larger airspace, which results in a stiffer fork. Since the compression of the airspace is gradual, lessening the airspace will be felt by the rider from the middle of the fork’s stroke to the point of bottoming. In essence, adding oil to your forks makes them stiffer from the midpoint on. The obvious corollary is that when you take oil out of your forks, you make them softer from the midstroke on. Adding or subtracting oil has a negligible effect on the first four inches of travel.” -Motocross Action Magazine
So if we break this down adjusting oil height is used to primarily tune the mid-stroke to the end of the stroke. The higher the oil level the more stiff your suspension will be from the mid point to the end of the stroke. The lower your oil level the softer your suspension will be from the mid point to the end of the stroke. You can add and remove oil from your forks through the bleed valves.
MXA Recommends making adjustments in 10cc increments and the best part about this mod is that it is relatively free besides needing a bottle of fork oil and a syringe. I bought this Fork Oil Syringe from Noleen Suspension Technologies for $9.98 and a bottle of Maxima Racing Fork Fluid for $12.99. These two purchases allow for lots of tuning for cheap. I do recommend your suspension being serviced by a reputable shop to your abilities and weight but sometimes you feel good and just need a little adjustment to make it perfect.