the black idler-wheel is much "friendlier" for the spurshaft. The white ones tend to eat the teeth on the spur-shaft away.
I always ran black idler-wheels ... but have now one black on the spurshaft-side, and one white on the diff-side.
My gear-case-problems have allready eaten away about 5 idler-wheels so far. (running on my last idler-wheels now
) ... but other then that, the car is superstrong.