OK, time to get on the soap box.
The issue with oil life isn't necessarily Viscosity. The oil is far from "worn out" when you change it. The thing is the acidity of the oil goes up as it gets older due to byproducts of the combustion process. Your oil not only needs phosphorous and such for wear but also calcium to neutralize the acids. This is why you change your oil before you lay up for the winter.
The other issue is your bike has a wet clutch, it is cooled by the oil in the crankcase. the newer automotive oils are (or can be) too slippery from the use of "friction modifiers" and cause your clutch to slip in some cases. These are required by the newer API standards for "fuel efficiency".
http://www.amsoil.com/products/aff.html
Scroll down to the part about friction modifiers. Here it mentions clutch compatibility and slip though it doesn't go into detail. Motorcycle Consumer News did a very in depth study on oils and their composition a while back covering all this.
If you scroll all the way down, the last number on the chart is TBN(total base number) This represents the ability of the oil to neutralize acid, the higher the better. If you go back and look at the Diesel engine oils you'll see this number is higher therefore prolonging the life of the oil in hte more acidic diesel process.
"In addition, the advanced 12 TBN additive package of AMSOIL Diesel Oil neutralizes acids and keeps the engine clean and in top-running condition."
Here's the home page
http://www.amsoil.com/ Look under product info.
Most motorcycles call for SG rated oil. SJ is a newer standard and exceeds SG(or SH) But isnt necessarily better. The newer standard is for
lower phosphoruos content due to the use of catalytic converters. Phosphorous is a wear agent and your bike doesn't have a cat/conv.
Basically, If your gonna use Wal Mart auto oil in your bike, your probably OK if you cange it a little more often. As long as your clutch doesn't slip any and you aren't cooking your oil in stop and go traffic in that air cooled motor that doesn't have a temp gauge. How do you know you aren't having thermal viscosity breakdown? My ZX12 is liquid cooled and it get
HOT sitting at a light in the summer. I won't chance it. FWIW
For the record- I am an Amsoil dealer, simply so I can get it for dealer cost for myself and a few buddies who have seen the light, no profit. I don't have the spare time to be an oil salesman, but I have a bud that makes as much as $1200 a month during snowmobile season. I run it in my cars engines, auto tranny fluid, bikes, 2 stroke for the sleds, etc. The only problem with the stuff is it's sold & distributed like Avon or Amway for the most part.
AS far as just changing the filter, I wouldnt do it. Here is the clip on service life for Amsoil synthetic as an example.
"SERVICE LIFE
In diesel engines, fleet vehicle engines and industrial engines, such as stationary gasoline engines: drain oil at intervals up to three times as long as those recommended by the engine manufacturer as long as extended drains are supported by the findings of a used oil analysis program.
In personal cars and light trucks with non-turbocharged gasoline engines and marine craft: drain oil at intervals up to three times as long as those recommended by the engine manufacturer or at one-year intervals, whichever comes first.
In turbocharged gasoline engines: drain oil at intervals up to twice as long as engine manufacturers' recommended drain intervals or at one-year intervals, whichever comes first.
In high performance and racing engines: change oil based on the findings of oil analysis.
In motorcycles, ATVs and related gasoline engines: drain oil at engine manufacturer's recommended drain interval."
Now if they say run fleet vehicles up to 3 time the interval but motorcycles at recomendedintervals there has to be a reason.
One other thing- once your filter gets dirty enough there is a bypass valve that will open so your engine wont starve for oil, but then all the dirt gets through. Change the filter too, cheap insurance.
OK, I'll shut up now.
:z