Haven't done anything on the suspension yet, the motor/ESC doesn't run smooth anymore since the BEC died, it's stuttering, even with external BEC. Bruno wants me to send in the motor and ESC, but haven't had the motivation to do so yet. I'm getting fed up with wrenching lately :)
Regarding diff fluids:
From what I have read, for onroad straight-line hard acceleration it's better to go for a quite thick diff fluid in the rear because it helps to maintain the same wheelspeed on each side. With thin fluid it can happen that one side has more grip and then the other side just spins which will throw the rear around as soon as it gets traction again.
I think I have experienced that effect when trying to achieve maximum acceleration from a standstill. When you look in the the video at 0:57s you can see how the car at first runs straight, then the rear steps out a little, I get scared, let off the throttle and after that the rear passes the front.
http://youtu.be/EVJkGJpTDKI?t=57s
At 1:46 in the video I managed to keep it straight, I think that was because I didn't let off the throttle which allowed the spinning tire to get grip gradually again (by the "natural" reduction of torque with higher RPMs).
http://youtu.be/EVJkGJpTDKI?t=1m46s
Always wanted to try a thicker fluid in the rear, but haven't done that yet. The diffs have the XO-1 stock filling with 100k front and 10k in the rear.