I use Valvoline 10w40 motorcycle oil. I never use synthetic because of the wet clutch friction needs. You can read about Valvoline here:
http://www.walmart.com/ip/Valvoline-10W40-4-Stroke-Motorcycle-Oil/16879749. I think the price was a little over $4 per quart.
Valvoline 4-Stroke Motorcycle Oils are designed specifically for motorcycles. Motorcycles place special demands on engine oil that automotive oils are not designed to handle. The wet clutch system, high RPMs and temperatures, and metals found in motorcycles require special properties for proper performance and protection. Valvoline motorcycle lubricants use premium base oils and advanced additives to bring out the best in your machine.
Valvoline 10W40 4-Stroke Motorcycle Oil:
Protects at high RPMs and temperatures
Protects wet clutch and provides maximum power transfer
Prevents deposit formation
Achieves maximum horsepower
Minimal wear
Advanced cleaning
Shear stability
Film strength
Excellent corrosion protection
Meets or exceeds manufacturer's warranty requirements
Provides exceptional protection for ATVs
For use in 4-stroke motorcycles, including cruisers and off-road motorcycles