T400 fan replacement

The fan in my T400 (type 2764-CTO) seems to be dying.
According to the nearest Lenovo authorized repair center, they will just ship the whole thing out and replace the mainboard.
Unfortunately its out of warranty so I figure that I would just do it myself.
I got it working with a little oil in the fan and have relegated it to desk duty.

Lenovo shows two part numbers for the fan; 45N6144 and 45N6146.
Does anyone know if there is a way to tell which one I need to get without taking the current one out, or does it even make a difference?