There's really no good way to lube those fans. They're designed with "frictionless" surfaces to run indefinitely (or at least for the stated lifetime, like 10K hours, or whatever) without a lubricating agent. What often happens is the surfaces wear enough that the close tolerances are no longer maintained, the rotating part wobbles (causing audible noise) causing more wear, etc. until the unit eventually fails. Sometimes one of the surfaces gets damaged by a piece of grit or such and that accelerates the damage. Adding a lubricant is only a temporary "fix" at best. The lubricant will get contaminated with dust and debris and thicken and perhaps even burn onto the surfaces as a glaze that is difficult to remove. A noisy fan is generally the first sign of the imminent failure...