Pinion on all diff gears are the same, for 41,40,39,38 and 37.
You are aware that they are available in steel ?
Medium traction = don't use high numbers on diff oil, 7k or 8k sounds right.
Technical (meaning lots of corners ? ) = one way in front
Long straight = gear the car correctly for your engine, you have plenty of option with all the sizes !
ps. Never use same oil in front and rear, always use a little thicker in the front !
I drive mostly on very technical track, high grip, not a very long straight.
Use 39 diff gears, 10k oil in rear, one way front. (Have the OS TZ engine, 3 ports stock). Played a lot with setup's for diffs, but in the end I stayed with the one way, love the exit speed in the corners (because you go in faster), and the traction on the straight.
Be aware, I'm no expert, nor am I a championship driver.. Just drive (alot) for fun. And learned a lot on the track from those fast drivers
Now I'm one of them !