The WOT operating range is probably listed at 5000-6000, though I don't have the service manual for your motor. When you get your manual it will be listed on the specifications page.
I think 5500 - 5800 would be ok target for the top end on a crossflow, but this motor is a looper and you might want to go say 6000 to 6200. It isn't about fuel quality, it's about lugging the engine.
See if you can raise DHadley. He can give you a more definitive answer. There is a point where you start to lose your advantage. I've been taught to peak at 200 RPM above the specifications range.
