What type of oil should I use?

What type of oil should I use?
"How to understand and choose Motorcycle Motor Oils". http://www.motorcycleanchor.com/motorcycle/how_to/mc_oil.html You won't go wrong with Castrol GPS or Mobil 1 MX4T.
BUT far more important is what kind of oil you should NEVER use:
Never use any oil which is marked "Energy Conserving" in the lower half of the API Service Ring (these oils contain friction modifiers that can coat your clutch and make it not grip);
Never use any oil which is API SJ, SL or SM rated as it's sole rating. Your motorcycle is designed to use an SF/SG and/or JASO-MA rated motor oil, which by definition conflicts with the API SJ, SL and SM standards. There are now special versions of API SJ and API SL motorcycle oils that can be used -- but these will have a JASO-MA (or JASO-MA2) rating on them.
Katana Oil Quick-Reference Chart http://www.motorcycleanchor.com/pics/katana_api_oil_guide.gif

How often should I change the oil?
Every 3500 miles at minimum, more if you do a lot of city driving or if the area is dusty (deserts, sandy areas).
What's the Right Way to Check the Oil Level on Katana?
Oil check method for Katana's (see owner's manual if in doubt):
Place bike on Centerstand if present.
Start bike
let run 1-2 minutes (to fill the oil filter, oil cooler, tranny, upper galleys)
shut off and wait 45 seconds (minimum) to 3 minutes (max).
Now with bike held straight up (or parked on centerstand if present), check window level.
add as necessary.