I use the Amsoil 10/40 that Amsoil recommends for our bikes. I change every 4,000 to 5,000 miles. Or 1 year...but it never gets that far.
From Amsoil's website:
SAE 10W-40 Synthetic Motorcycle Oil (MCF)
AMSOIL Synthetic Motorcycle Oils have been reformulated with new high performance additive technology that provides even greater multi-functional benefits for the special requirements of motorcycle applications. These independent and exclusive new AMSOIL formulations provide second-to-none viscosity protection for hot-running American and foreign motorcycle engines, transmissions and primary chaincases. Performance specifications include:API SG, SL/CF; JASO MA/MA-2; API GL-1
APPLICATIONS
AMSOIL Synthetic 10W-40 Motorcycle Oil is recommended for liquid or air-cooled 4-stroke engines. It meets SAE 80W/90, API GL-1 gear oil requirements and is recommended for transmissions on both 4- and 2-stroke motorcycles. AMSOIL MCF is recommended for Honda®, Kawasaki®, Yamaha®, Suzuki®, BMW®, Husqvarna®, Victory® and other motorcycles where 10W-40 or 20W-40 engine oils or SAE 80W/90, GL-1 gear oils are used. Not recommended where an API GL-4 or GL-5 gear oil is required.
SERVICE LIFE
For "on-road" use in engines and transmissions, change AMSOIL MCF and AMSOIL engine oil filter at twice the motorcycle manufacturer change interval or one year, whichever comes first.