Add this to some of the oil threads. From a MOTOR magazine article. While written from an automotive POV, seems some thing would apply to motorcycles. Lots to read here.
http://www.motor.com/newsletters/20110523/WebFiles/ID2_Viscosity.html"By definition, viscosity is an oil’s resistance to flow and shear. It is arguably the single most critical physical property of the oil as it affects both the wear rate and fuel efficiency. For motorsports, winning races is more critical than engine life, so high-performance engines may use lower viscosity oils and parts with expensive low-friction coatings to maximize horsepower output to the wheels. For everyday street use, however, engine life is the more relevant metric, so following the original equipment manufacturer’s (OEM’s) recommended viscosity for an engine matters..."
"... Many VI Improver additives are susceptible to shearing damage from engine mechanical components and can lose some of their performance, resulting in 20W50 oil acting more like 15W-40 oil..."
And this I found interesting...
"Synthetic oils typically have a naturally higher viscosity index than even the artificially increased VI of mineral oils."
