I run car oil in my '89 zx10 and my TTR250, but I change it frequently. Both these engines were designed before motorcycle specific oils were created.
A motorcycle engine gives oil a much harder time than oil in a car engine. This is because of the wet clutch and gearbox. As stated earlier modern bikes need the JASO spec oil this is the link which tells you all about it
http://www.jalos.or.jp/onfile/pdf/4T_EV0604.pdf
I always use a decent motorcycle oil in my Tiger.
Chris
