I suspect that the US oil companies don't trust oil with that broad a viscosity range at the temperatures encountered in a motorcycle engine. The way they make a wide range oil is with a long polymere chain that rolls up when cold and unrolls when hot. The longer the chain, the broader the range, and the greater the chance that it will break down in adverse conditions.
If cost were not a factor, you can find the very best oil for use in aircooled engines at you local airport service shop. Aircraft engine run for hours at a time at 90% power in a wider range of conditions than any sane person would ride.
