[Jacke]

Lately the fan on my eMac has been really noisy, noisier than usual. So I was thinking about dropping a drop of oil there to see if it would help, and was wondering if anyone else has experience with that sort of thing? For example, is it enough to just remove the plastic case, or should I also remove the fan and drop in the oil on the side facing inwards?

[C.A.T.S. CEO]

Replace the fan. To my knowledge the fans inside eMacs (and all other computers for that matter) use magnets to decrease friction for better efficiency, putting oil in it would most likely screw it up even more.

[Jacke]

I might be able to get a fan from an old first gen. eMac, do you think that could work?

[Sherman Homan]

CATS is right, the bearings are totally sealed. No oil will get in there. However, you might succeed in having the fan distribute a fine mist of oil droplets all over the inside of your machine...
The fan is going to have a physical shape (form factor) and a cubic feet per minute air movement rating. If you can match those, you should be fine.