Heavier oils, and synthetic oils especially, I believe, have much greater shear strength than light oils. So, just for the sake of debate, you might speed up oil delivery a bit with a light oil, but you lose some shear strength. More quantity, less quality. ??
Having said that, I suppose it could be true that roller bearings don't require the shear strength that a plain bearing engine would.
I tend to lean towards heavier oils, I'm just not convinced that lighter oils flow that much faster, or that it makes that much difference with a roller bearing engine. I think our imaginations tend to get carried away thinking that heavy oil flows like gear oil or maple surup. With plain bearings, I think that's probably more true, or more important, to get oil to the rod and main bearings faster. Synthetic 20w50 seems like a good compromise for me...but the last time I had a 45 was back in the straight weight days, and I'm just now putting a big twin flathead together...so I've not tried it yet. But that's what I'll start out with.
I think you might be missing my point. Heavy oil do not give lesser speed on the oil. And it do not contribute to slow down oil delivery unless the oil is so thick that the pump struggles to suck the oil form the tank and cavitates. However on flatheads the relief valve sends oil to the cam chest. Too heavy oil causes the relief valve to open early and sending more oil to cam chest and less oil to the rods and pistons. This is not the case om sporsters, because they do not have this valve.