Originally Posted by Brabus007
The second link doesn't look trustworthy to me because they are trying to sell their own product.
The first link does have a valid point, but only if you're always running your engine wrong somehow or another. They are talking about the oil film, which does get generally better with higher viscosity. If you're doing it right, then you're not relying on the film for separation. The separation forces other than the film
are defined by oil flow, not pressure. Oil pumps for cars are constant flow (per RPM) devices.
At high RPM, the dominant forces are from the reciprocating motion of parts. The oil flow increases with the RPM to match the higher forces those parts produce. If the oil is too thick, however, it may provide high enough pressure across the pump to activate the pressure-limiting bypass valve. Revving faster than that will increase the forces that need to be controlled, but not the oil flow that's supposed to be doing the controlling. Using a thicker oil will keep you from doing so much damage when you get into that mode of operation, but if you had used a thinner oil you may not have gotten that problem in the first place.
At low RPM, the dominant forces are from the events in the cylinders. These get bigger with higher load. The flow will be the same no matter what the viscosity of your oil, so thicker oil will lubricate better (better film). This is why low-RPM high-load operation is considered bad.
Simple version: thinner oil works better at higher revs, thicker oil works better at lower revs, if you know what you're doing so as not to kill your engine you can get better MPG with thinner oil.
Also, keep in mind the effects of temperature on viscosity.