While fans usually do a good job of cooling they also cause air friction over all the plastic parts including circuit boards. This creates a static attraction to dust and over time the amount of dust attracted will form an insulating layer that prevents cooling of possible critical components. While you are cleaning and oiling the fan do not forget to clean all the circuit boards with a non static brush. We use a good pig's hair or sable brush for this.
Regards
Neville@millennium3.ws