Viscosity is rated at ambient temperature I believe. At operating temperature, they all pour about the same like water. Higher viscosity oil increases friction. Friction causes heat. Oil temperatures will rise because of this heat. I will give you my testimonial. I used to run 20-50 mobile one in the VW. At Daytona, I would see 260F oil temperature and luckily these were short races. This was with an larger external cooler. I talked about this with Redline at the Circle Track trade show. Their simplified explanation was that the additives and the viscosity were causing the higher temperatures. They suggested trying their 40WT racing and if the pressures still looked good to then try their 30Wt. Here is the testimonial part. With 40WT and no changes, the oil temperature dropped 20 degrees. Oil pressure remained as before. Next oil change switched to 30WT. Another 20 degrees oil temperature drop and hardly noticeable drop in pressure. We have used Redline 30WT in our GT cars ever since. I would not be afraid to try a lower viscosity oil. I would also not be worried about my bearing clearances even if your are pushing it to .003". Keep an eye on your oil temp and oil pressure. Less friction equal less heat and more hp. Sounds like a no brainer. As a reference, 10 pounds of oil pressure for every 1000 rpm will keep you together.

In my opinion, bearing failures are often more related to other problems, detonation and cavitiation. No oil can compensate for that type of mechanical problem. Good luck.