I have an '02 Dyna Service Manual, and it calls for 12 oz (if left in the frame) of Type E Fork Oil for the FXDWG. Not sure if its the same for the '04, but thought I'd mention it. You can get an ebook version of your specific service manual on the internet for cheap. I bought a new owner's manual for mine on ebay for about $13 shipped.
I used a cooking measuring cylinder when adding mine. It had a static measuring feature so that it was pin point accurate to have the same amount in each leg. I also used Type B Fork Oil by accident, but it worked out pretty well and I actually like the ride better.