Ahh the oil debate. I may sounds like a broken record, but Enthalpy's motor always ran Mobil 1 10W30, and that thing was spotless when the rings went. I ran the same in my motor at 9k+ rpms and never had any oiling problems. That's why I use Mobil 1 now, even though the new engine is way less stressed.

The heavier oils like these 20W50 diesel oils are what I've seen the extreme duty drag race guys use. A heavy oil will withstand more load, and keep the bearings off the crank. The deal is, though, that race motors have huge bearing clearances; 4 times bigger in some cases.

I don't have any real evidence, just a train of thought/logic going here, but here goes. You've got an oil pump, which has a pressure relief valve. The oil pump will only ever hit 6 bar of oil pressure before the spring relief vents. With a thin, warm oil, that won't happen until 4-5k rpms. With a heavier oil, it'll happen earlier. Sounds great right? Yes and no. The difference is the mass flow of the oil. The heavier oil is denser, so at the same pressure, you're getting less flow. Less flow means less oil cooling the bearings. So, good at lower rpms, but worse at higher rpms. But, with more bearing clearance (like a race motor), the restriction isn't as big and you're allowing more oil to squirt past. You'll regain that oil flow past the bearings to address cooling, and that heavy oil will cushion better because of it's density.

I have no idea if any of that is right. Maybe someone who reads all of these oil threads can find a similar or differing statement.