Can you use a petroleum oil, of course.
Can you use a Synthetic blend, yes.
Can you use a Full Synthetic, yes.
Can you use 100% Synthetic, Absolutely.
Reasonable results can be had using a good quality petroleum oil and a good filter. Superior results can be had using a Premium Synthetic and a Premium filter.
Periods of Non-Riding are more harmful that riding. Acidity continues to build during those times and corrosion occurs on metal parts in as little as two weeks if an oil without anti-rust and corrosion inhibitors is used.
The acidity buildup is why an oil also needs to be changed within a time limit as well as a mileage limit.
AMSOIL, is the only oil I am aware of where you can double your mileage or up to one year of service and is safe to store on after the engine is run.
Enjoy that 650... I use to have one. Great little bike.
The Best isn't cheap
Cheap isn't The Best
Bob