I just looked at my Clymer manual, and a older HD manual they don't say anything about how much the rear hold.
A little oil goes a long way making a mess. Unless you broke a hose and ran a long way I don't think you need to worry about it. Like the forks only hold 9.7 oz for 99 to 2001 and 2002 is 11.1 oz and a rear shock is a whole lot smaller.
They can be refilled thru the air fitting, hard thing is to figure how much to use. If it only came out of one you can measure what is in the other one. If they leak at the seal they are history.
Harley has no info, but the Honda dealers at one time had the Showa fluid they take, or you can just use fork oil and experiment with weight to where you like it, but don't mix the oils just in case (empty them first as much as possible)
Not sure anymore but it came up once before and I think it was 10W.