I used a JC Whitney HD chain for nearly the first 10 years I had my 650. It worked fine and served me well. When it came time to replace it, I went with something a bit better, a D.I.D. standard chain (not o-ring). The D.I.D. is supposed to be one of the best standard chains out there and can be had for around $40 if you shop around. Personally, I don't feel the need for a fancy, expensive o-ring chain on the 650, it's not that big and powerful.
Hi 5twins,
the cheaper the chain the quicker it and it's sprockets wear out.
Renolds tech pages say industrial #50 chain is only rated at 5HP.
I reckon that to transmit 50HP a #530 bike chain needs to be the best one you can get.
My XS650 is running a #530 O-ring chain and is lubed with a ScottOiler.
I expect it to outlive me.