Being air cooled motors, the oil is used for lubrication, but also for some amount of cooling too. If you spend a lot of time in stop and go traffic, city traffic, or run your bike hard, I would change oil more often than what that manual suggest.
In all the Kawasaki motorcycles I owned or had as demos I NEVER ran the oil and filter more than a 1,000 miles before I changed it. Most of my friends that still have Kz's in their collection would never think of running more than a 1,000 to maybe 1,500 miles before they change it!!!!! The factory Porsche shop manuals for the air cooled flat 4's and 6's (356, and 911 models) recommend the oil be changed at 1,500 kilometers. Which is a bit less than 1,000 miles I believe!!!!
This is the Spectro oil I have been running since back in 1986. My Kz1000 motors show ZERO wear on cams, shim buckets, or cam bearings!!! On my GSXR750 motors, ZERO wear on the cam lobes and the rocker arm pads!!! Which are a major wear problem on that type of motor.
Good $hit Maynard!!!!