Oil film bearings do not rely on the oil pump to lift the surfaces apart, like a hovercraft. The bearing itself draws oil into a wedge shaped zone (formed by the differing curvature of the two surfaces). More is drawn in than can pass through, so some has to do a U-turn and flow back the other way. This happens because the pressure rises very high (to make the fluid do the u-turn). This induced pressure is what holds the two surfaces apart.
How far apart they get depends on the load and the pressure. More viscous oils develop higher pressures, and therefore separate the surfaces more, for a given bearing and a given load/speed. All of this heats the oil tremendously, so the oil pump flushes out the hot oil, replacing it with cool oil for the next revolution. More viscosity=more heat and less flush flow, so you can have too high a viscosity.
I've mentioned viscosity as if it were fixed. Most oils are very Newtonian (the viscosity is independent of shear rate), but not totally. When the film gets very thin, long chain molecules reptate. Reptation is a term that refers to the chains lining up against one another in thin films (like snakes, hence the term). When they do this, the behavioral viscosity goes WAY up. When not in thin films, the chains curl and overlap, and the observed viscosity goes down.
This process is why oil with a viscosity the same as water lubricates much better than water. We use terms such as film strength and lubricity to describe this phenomenon.
Synthetics are made from uniform-length long chain structures..no aromatics, very low branching. More and longer chains results in better reptation and higher lubricity for a given "in the bottle" viscosity.
Without reptation and the resulting non-Newtonian behavoir, we'd need to run multi-hundred weight oil in the crankcase. It's also why molasses (which are high-viscosity) would make poor lubricant...no film thickening.
Shear stability is the ability of the molecules to not break apart when sheared. All this high shearing can (and does) mechanically break the molecules. This causes a loss in viscosity and an even larger loss in lubricity. Smaller molecules don't lubricate, because they don't reptate as well. Syn molecules are stronger, and can therefore get beat up more without breaking. It's one of the reasons they last longer.
Strictly speaking, higher viscosities do lubricate better, in that the oil films are thicker for a giver service, but not at the expense of thermal stress. Your oil pump is intended to dose all the bearings with fresh cool oil each rev. If the viscosity is too high, it can lead to oil flow bypassing the system through the pump pressure pop-off valve, bypassing the filter due to high DP, and mal-distributed flow through the journals. Because more heat will be generated in the bearings, they need more flow, but the high viscosity leads to less flow, so there's more heating and more bearing drag.
That's why bearings are designed (surface area, gap, finish, etc.) for a given oil viscosity in a given engine. More lubricity is better, not more viscosity.
Viscosity also affects heat transfer coefficient...not thermal conductivity, but rather film coefficient. It's harder to remove heat from thicker fluids by contacting with cool surfaces.
