Fork oil level makes a big difference in how easily the supension bottoms out. The higher the oil level, the stiffer the fork gets as it gets close to bottoming out.
Oil level is measured from the top of the fork tube, with the fork springs removed and the forks compressed all the way.
My '84 stock had the oil level at least 6" from the top as measure above. Add enough oil to raise the oil level to 4" from the top and that should get you in the ball park. You can alter the oil level to fine tune the suspension.
You should use about 10w fork oil. Oil weight mainly effects how quickly the forks rebound. With your soft stock springs, don't go too heavy on the oil or the fork will rebound too slow.
If your forks have not been serviced in a long time, you can drain the oil out and replace with fresh oil. Rather than relying on the volume in mililiters specified in the manual, measure by height from the top of the tube (with forks compressed and no springs in, of course).
BTW, if you are removing the fork caps, be sure to loosen the pinch bolts that hold the top of the fork tube in the triple clamp. If the pinch bolts are tight, it will squeeze the fork tube around the fork cap and make the cap hard to remove.
Also, if you are on a tight budget, you can slightly stiffen your stock springs by cutting off a few coils and then adding length to the pre-load spacer using a stack of washers or a peice of metal tubing. Basically, cutting about 10% of the coils off your stock spring will stiffen the spring by about 10%. This helps a lot on an '84 KDX. Don't cut too much, or the fork will lose travel.