Start the bike up and let the oil warm up (side of the cylinder should be pretty warm to touch) before killing engine. Remove drain plug (on bottom of engine) and let all oil run out. Replace drain plug and remove filler plug on side of cases and add oil.
.7 L
KTM recommends Motorex Top Speed 4T15W50 for the 125SX.
my 03 manual says for the ktm sx 125 20w-40 .7 liters.
.7L
.7 L
You can find that info , as well as a lot more, here: http://motocross.transworld.net/2003/07/09/2004-ktm-125-sx-200-sx-250-sx-and-525-sx/
750 ml per leg
depends what climate your running it at. 30:1
800ml
.7 L
My manuel recommends 40 to 1on my 2001 ktm sx I've never had any problems with it..
KTM says to change the transmission oil the amount called for is 0.7 liters engine oil using Motorex Top Speed 4T 15W50.