I'll agree that more flow should give better cooling.
But doesn't the thicker oils resistence to flow mean its trapped in the bearings giving a thicker cushion of oil?
I imagine it like a bike with a leaky tire for that for some reason is filled with oil. This bike has an oil resevoir and pump that can fill the tire as you ride. The thicker the oil, the slower the tire leaks giving higher tire pressure. Thinner oil leaks out faster giving the tire lower pressure. Either oil in the tire is fine for riding down the street until you hit a sharp edged pothole, which oil do you want to have in the tire to avoid denting the rim? Thicker higher pressure oil, or thinner lower pressure oil? I'd rather have more pressure in my tire just in case...
This little analogy ignores hydraulic locking effects of forcing oil through a little hole quickly but for me it shows the advantage of thicker oil in a motor. What if your engine pings for some reason? What do you want in the rod and crank bearings, 5W20 or 15W40?