I've been told synthetic is good as long as it doesn't carry an energy conserving label.
Energy conserving indicates the presence of friction modifiers that can allegedly damage your clutch.
Only engine oils of up to W-30 viscosity can be energy conserving rated. Friction modifiers which are not wet clutch compatible are still found in some automotive W-40 and higher viscosity oils. Motorcycle specific oils are JASO MA certified for high friction applications, as recommended by Kawasaki.
From Castrol; "Note: The low friction characteristics of Castrol Edge 0W-40
make it unsuitable for most motorcycles that incorporate wet clutches in combined engine and transmission systems. Castrol’s prime recommendation for this application is Castrol R4 Superbike."
I use Amsoil synthetic AFF 0W-40. You can get any Amsoil product from http://www.woodsbrosracing.com/amsoil-online-store.htm
at over 20% off with promo code.
There's also Shell's excellent Rotella T6 synthetic 5W-40 that's available for less than motorcycle oil, as it's not JASO MA labeled, but still passes the tests. According to Richard Moore, Staff Engineer at Shell Global Solutions (US) Inc., Westhollow Technology Center, Houston, TX (800-231-6950):
"We recently ran the JASO MA friction test on Rotella T with Triple Protection 15W-40, Rotella T Synthetic 5W-40 (CI-4, discontinued) and our Rotella T6 Synthetic 5W-40 CJ-4. All three oils passed the wet clutch friction test. Rotella T Synthetic 5W-40 (CI-4) has more than 1.2% ash (JASO MA spec limit) so it can not be classified as JASO MA. However, Rotella T with Triple Protection 15W-40 and our Rotella T6 Synthetic 5W-40 CJ-4 do meet JASO MA."