I have never tried Royal Purple, but enough people like the oil or its price that the company has stayed in business for many years.
A lot of oil choice is how you think it makes the clutch and shifting feel. You might like its slightly different feel better than someone else. The Advance Auto Parts store near me sells the car version for a good price, and any hot weight above 30 is not labelled energy conserving and should be fine for most if not all clutches that are not on antique bikes.
Below are some quotes from Anyone use Royal Purple in their Bike(s)? - Motorcycle Forum
I've never been impressed enough with a used oil analysis of Royal Purple to put it in any of my engines. If you go by the UOAs on Bob Is The Oil Guy.com, Mobil 1, Pennzoil Platinum, and Amsoil all outperform it.
I've been running Royal Purple in my '09 Yamaha V-Star 950 for about 1200 miles now (since my 4000 mile oil change) and have noticed better shifting and a quieter engine.
from an Amsoil rep: Take a look at how RP performs in the Motorcycle White Paper which was conducted by several of the Top Testing Facilities in the US.
soxOZ has a point.. Oil has 4 Major functions. Cool, Clean, Lubricate and Seal an engine.
A synthetic oil will often increase the time before the thermostat opens, or increase the time it takes before the hot coolant warning light comes on compared to a conventional oil, especially for sportbikes that are designed to be lighter and do not cool down well when going slower or stopped.
Mike Guillory wrote this: "All 5w40, 5w50, 10w40, 15w40, 15w50, and 20w50 oils which I have found are not energy-conserving and can be recommended for general motorcycle use."
Your Kawasaki owners manual probably includes a range of oils that the engine can use, depending on the temperature. If you want to ride the bike for 300,000 miles without a rebuild, using a 20w-50 weight oil in the summer and a 10w-40 weight oil in the winter might increase your chances.
I have not been brave enough to try this super-light oil below- many drag racers use it and they are very hard on engines, but they also change the oil a lot sooner than for a street bike. See the last 2 paragraphs here:
Brocks Performance: Bill Warner Featured in Race Engine Technology Magazine