Another thing to take heed of is how much traction and/or bumps/holes the track has. General rule of thumb:
Loose track, drop the diff weights a little all the way around (maybe drop a hair more in the center compared to the other diffs) will make the car easier to drive.
Same is true for the opposite:
Track with a lot of grip, raise the diff weights just a little (maybe raise a hair more in center compared to the other diffs) to make the car respond more to the grip.
Lighter diff oils will be easier to drive in bumps while you will want heavier diff oils on smoother tracks to get more rip.