I thought it would be helpful to some to put together a quick but informative instructable on how to take a case fan (or any computer fan for that matter) apart down to the bearings and motor assembly.
This way you can soak/clean/wash the fan blades without risking damage to the pcb from immersing it in liquids. You can oil it too afterwards if you like.
And yes, this applies to all computer fans of all sizes, however be warned, the smaller computer fans (such as those found on GPU's) are a lot trickier to pull the ring off to remove the fan blades.
Step 1: Remove the restriction ring
As you can see, you want to remove the white plastic ring from around the end of the axel. A pair of tweezers or the tip of a thin pocket knife also works, however be careful not to break the ring, as you'll need to for later when you put the fan back together.
Also, some case fans have metal clasps instead of plastic, I'll try to get a picture of one later this week.
Step 2: Clean the fan
Now for the fun part, everyone has their own methods, and depending on the build up it may need anything from a blast of air from an air compressor to soaking it to remove the caked up gunk
This isn't something I recommend doing a half-ass job on, as you'll only be taking it apart a month later because all the dust you left on the fan will collect even more dust, especially if it's still wet when you start using it again.
I myself soak the fan blades in a secret blend of hydrochloric acid and rubbing alcohol (I'm only joking about the former).
Step 3: Re-assemble
Now that you have your fan all spankin clean, it's time to put it back together. I couldn't find my oil today so I didn't include the picture/indivigual step for it. However note in the pictures below where to put a couple dabs of oil.