I use 3 quarts even when I change the oil and filter. 3.1 is prolly the "correct" amount as someone else listed. It is usually up over the full mark with 3 so that is what I use. If not, try 2.5 and see where you are at, either way you are going to have to buy 3 quarts. Just change the filter and be sure of it.
actually.. if you look at the manual, make sure you check because it should be 3.1 for a bike w/ filter change. Also you want to always have the best filtration performance, and therefore it's worth the 12 bucks to change it.