The viscosity of the oil will break down over time and produce less damping action and needs to be replaced because of this. At the same time as you change out yourfork oil, you need to replace the wear rings in the fork as well as the dust seal and oil seal in the fork.
The oil serves a couple of purposes in your forks. It provides lubrication for the sliding parts, keeping them from wearing metal to metal. The oil also provides your damping. The viscosity of the oil will break down over time and produce less damping action and needs to be replaced because of this. At the same time as you change out your fork oil, you need to replace the wear rings in the fork as well as the dust seal and oil seal in the fork. A good cleaning and new fork oil and you are good to go for some time.