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 don't know if it's true, but I've been told that a Harley lower end can run quite a while with the oil pump out, without being damaged. I don't think the pistons and rings would like it though! ?? Don't know if that's true.
I'm thinking that it was suggested to add kerosene to oil WAY back in the day, when people were using side car rigs in very sub-zero temps. I bet that not too many antique Harley riders are going for joy rides or on runs when it's below freezing, ten degrees out, 20, or even 40 degrees out for that matter. So I'm not too sure that lighter oil for colder weather holds true for us flathead enthusiasts.
And, I could be wrong. !!