that's perfectly normal. It's just a light coating of oil on the inside from manufacturing.
http://www.ninjette.org/wiki/Lean_or_Rich
Here's the technique for jetting.
with just a slip-on, you should be fine to just shim the needles out a little. Desnorkel while you're at it so you only have to adjust your jetting once. But still get a pair of jets that are 1 step larger than stock, and do some WOT tests to make sure the top end still feels good before you mess with the needles.