How much oil ?? Or just measure what you drain..
HD spec say 12oz. I always measure what I drain out, typically ~11oz as you can't get it all. I have drained as little as 6oz, good HD quality control. I always put 11.5 oz in the measuring cup knowing with that I will get 11oz into the shock.
You can't drain them, well probably could but it would take forever. The best way is to pump it out & back in using the shock itself.
With the 10w Amsoil I run only 18 lbs of air 2 up under normal conditions. Fully loaded on a trip, with the trailer I only run 25lbs. This lets the shock actually work.
Now, this is only an inexpensive method, I am sure there are much better shocks out there but this works for me. It took all the diving in the corners away, nice & controlled thru the corners now.
Here is a link with pictures of how to change the oil.
http://tinyurl.com/3b5jn5lHope this helps