API SJ is fairly new and already JASO has stepped in with their MA and MB certifications because of the large problems that the API SJ ratings have caused in regards to wet clutch slippage.  Castrol and Mobil are huge automotive oil manufacturers as you know and they have recently entered the US market with their motorcycle specific motor oils.  They also recommend to use motorcycle specific motor oils and not auto oil for motorcycles.  If you compare the price of MOBIL 1 for autos to their comprable full synthetic oil for motorcycles you will see a huge price difference.  Rightly so because it is not the same oil in a different bottle, it is a special formula designed for motorcycles and it goes way beyond the clutch issue.  You'll find the same true with Castrol.  The OEMs like your friends at Kawasaki have already got on the JASO MA band wagon and are recommending it for motor oils with the API SJ approval.  Unfortunately the JASO spec only relates to clutch slippage but what about the other issues like the high anti-wear properties of motorcycle oils or the high flash points because they must work in air-cooled, oil-cooled and water-cooled engines.  High flash points mean no oil volatility.  What about the polymers used that can withstand the shredding effects of transmission gears so you will hold your viscosity rating.  I could go on and on.

The fact is most people that own street bikes at least in the US use Castrol GTX auto oil found at just about any grocery store, Wal-Mart etc, and its very inexpensive.  Will your bike blow-up and the clutch smoke, etc.? Probably not, but you may or may not experience clutch problems, many have.

The average guy doesn't care and doesn't ride that much.  We manufacture high performance specialty lubricants for motorcycles, the average guy is not our customer.  Our customer logs many miles or in your case kilometers and wants the best for his bike in performance and protection. 

Below is the #1 reasons why e recommend our finest oil Bel-Ray EXS Synthetic Superbike Oil 10W-50:
EXS can handle extreme heat and will dissipate heat better, its flash point is over 465 degrees F (240 C).
EXS will give you cold start-up protection because it molecularly bonds to metal surfaces.
EXS will give you extremely high anti-wear properties and film strength.
EXS is designed to be used with a wet clutch and will cause no clutch slipping or grabbing.
EXS exclusive polymer system can withstand the shredding effects of transmission gears and hence hold it's viscosity for the long haul.
EXS can go further with longer oil change intervals than petroleum motor oils.