Well having seen some fans that used grease that pushed out of the bearing quickly I wont use it. I personally still use a good oil. NON detergent motor oil or my fav oil i get off of ebay.. liquid bearings. Oil floats the shaft on most fans unless there magnetic bearing fans then don't use any oil. When you turn off or even when running the oil flows back in and keep the metal in the riffle bearings from contact. Grease is to thick to let the fan turn at full rpm and cause extra heat that may let it turn faster but it will in time shorten the fan life.. a slightly thick oil is fine like non-detergent 40 motor oil. if you use 3n1 then reoiling is what you have to do every 6m minimum. never never never use a detergent oil as it will when evaporated leave gum behind.
here is a good short
The type of bearing used in a fan can affect its performance and noise. Most computer fans use one of the following bearing types:
* Sleeve bearings use two surfaces lubricated with oil or grease as a friction contact. They often use porous sintered sleeves to be self-lubricating, requiring only infrequent maintenance or replacement. Sleeve bearings are less durable at higher temperatures as the contact surfaces wear and the lubricant dries up, eventually leading to failure; however, lifetime is similar at relatively low ambient temperatures. Sleeve bearings may be more likely to fail at higher temperatures, and may perform poorly when mounted in any orientation other than horizontal. The lifespan of a sleeve-bearing fan may be around 40,000 hours at 50 °C. Fans that use sleeve bearings are generally cheaper than fans that use ball bearings, and are quieter at lower speeds early in their life, but can become noisy as they age.
* Rifle bearings are similar to sleeve bearings, but are quieter and have almost as much lifespan as ball bearings. The bearing has a spiral groove in it that pumps fluid from a reservoir. This allows them to be safely mounted with the shaft vertical (unlike sleeve bearings), since the fluid being pumped lubricates the top of the shaft. The pumping also ensures sufficient lubricant on the shaft, reducing noise, and increasing lifespan.
* Ball bearings: Though generally more expensive, ball bearing fans do not suffer the same orientation limitations as sleeve bearing fans, are more durable at higher temperatures, and are quieter than sleeve-bearing fans at higher rotation speeds. The lifespan of a ball bearing fan may be over 60,000 hours at 50 °C.
* Fluid bearings have the advantages of near-silent operation and high life expectancy (comparable to ball bearings), but tend to be the most expensive.
* Magnetic bearings or maglev bearings, in which the fan is repelled from the bearing by magnetism.
http://en.wikipedia.org/wiki/Fluid_bearing