my 03 manual says for the ktm sx 125 20w-40 .7 liters.
10w/40 semi-synthetic
10w 40 ATV oil
Fully Synthetic 10W 40 or 5W 40
10w 40
i would suggest 10w 40
10w-30 or 10w-40 oil
10w/40
No
use 10w 40 - 20- 50works the best
For the 2000 SX 400 KTM recommends SAE 10W-40 wt.; Capacity 1200cc See link below for technical specs.
My manuel recommends 40 to 1on my 2001 ktm sx I've never had any problems with it..