whats up eel? To change the transmission oil you must first:
a. Remove the oil fill cap ( It should be ontop of your clutch cover).
b.loosen and remove the drain plug that is under the transmission (underbike).
c. Drain oil and put drain plug back on.
I use ATF type F fluid in my Kx and it runs good. There are other oils though.
Bel Ray gear saver SAE 80w would be good for your bike. Also Maxima MTL gear oil 75w, 80w,85w.
I use Honda 80 weight racing gear oil. Because a Honda shop is the closest shop to me, other wise I would use what ever 80/85 weight gear oil the nearest shop had.
The reasons I use gear oil: There is a lot of controversy even between knowledgable people as to what oil is the best so I use what the manufactures manual says. After all they built is, they should know.
The place to put the oil in (filler cap) is just in front of the kick start lever. the amount to use is 850cc. the drain plug is on the bottom of the engine pretty much right below the filler cap. Becareful putting the plug back in as it is very close to the frame. Mine actually touches the frame when re-installing and I have to play with it to make sure it is threading properly and not cross threading because it is touching the frame. DO NOT FORCE IT. Hopefully yours does not touch and will go right back in no problem.