MY First KDX - how much fork oil

Not sure about the oil volume, but with the springs out and the forks compressed, fill to about 110mm from the top. Oil level effects the last part of the travel and is mainly used to control bottoming resistance. Too little and you will bottom easily, too much and you won't use all the travel. It is fairly easy to add or remove some later, but 110mm should be close.

While you have your forks apart, I strongly recommend Race Tech cartridge fork emulators and stiffer springs. Unless you are very light weight, the stock fork is too softly sprung. It also has too much high speed compression damping. The result is it is wimpy on jumps and whoops, but harsh on rocks and roots. The emulators and stiffer springs make a HUGE difference on the older KDX.

If your budget does not allow for emulators or new springs and you still have the stock, soft springs, they can be stiffened by cutting off a few coils. If you do this you will need to add some length to the pre-load spacer. With only the spring change, the bike will be better on jumps and whoops.

Here is the recipe I use in my '92 "E" model.I also have springs out of a later model KDX which are a little stiffer.At 160 lbs. I don't need much.
For '89-'92 conventional forks Freddette says to remove the spring and compress the fork and then pour in the oil until it's 4.75 inches down from the top of the fork (for 200 lb riders. 5" for 150 lb riders). He says to mix 5 and 10 wt fork oils together for this.

My manual says 560ml of oil.