Unless you are doing a total dis-assembly of you fork, the manual stated quantity of oil isn't going to do you much good. There will be some oil left in the tube and if you add the the 15.9 oz (per side) you will have over filled.
I just did the forks on my '08 KLR last night. I didn't do a complete R&R on them because when I pulled the caps, the oil looked like it was brand new. So I just put the bike on the lift and pulled the caps, then lowered it down until the forks bottomed out.
Then I removed the spacer, the washer, and the spring from one side. Using a piece of 3/8" x 96" clear aquaium pump hose I started a siphon going using the same method as you would to siphon gas from a car. The fork oil pulls slowly so I let it drain into a low pan for about an hour.
After all (or most) of the oil had been siphoned from that side of the forks, I replaced the spring and washer, and filled that tube until if was equal in oil level to the other side. In this case the oil just covered the hole in the washer.
Then I did the same thing to the other fork tube. On this bike the oil level is suppposed to be set at 7 1/2" below the top of the tube, but it is often difficult to measure this height. That's why I did the comparison fill with each tube....it was easier to see.
If you have a vacuum pump it's a lot easier. I don't have one so.....You can also use a turkey baster with a proper length hose on the end, but the siphon way works well, though it is time consuming.
Fork oil level is critical, but it can be altered to obtain different handling needs. Inside the forks the air trapped in there when you put the caps on, acts as a spring. The more it's compressed the more it resists. Put in more oil (or more air on air style forks) and this air compresses sooner, effectively giving you a stiffer ride. Fill the forks up to the very top with oil and your forks won't work at all....no air to compress. lol
Anyway, I would pay closer attention to the oil level than I would to the specific volume oil, especially if you leave the forks on the bike. You will use about half a quart on each side. Probably just enough more than half to require you to buy two quarts. lol
Remember, the fork oil level is measured with the fork completely collapsed. And you want both sides reasonably identical.