I'm assuming your asking about topping up as you haven't taken the forks off?
Personally if you've gone to the effort of new springs and spacers then new oil would complete the circle especially as dirt drops to the bottom of the fork.
If you don't feel your ready yet then what I have done on others but not yet a bird, is once you have the spring/spacer out then I've used one of these to get most of the oil out
for some reason the link isn't working so go to halfords webs site and type in laser syphon pump, it's £2.99
Once you've got out as much as you can then fill with 10W fork oil
if however your simply going to top up if you feel the oil is good, your choice, then 10W fork oil is what you need
This thread and post 16 might help you
http://www.bikersoracle.com/blackbird/forum/showthread.php?t=65890&highlight=fork+oil&page=2