Oils and bearings
Diagram 1

 Understanding oil pressure, oil flow and surface pressure vs clearance is the first step to understanding how and what the oil's job is. Not only is it there to lubricate but also to cool. 

Around 30% of an engines temperature is controlled by the engines oil system and depending what you are building for eg: drag racing, circuit or endurance will effect where you want the oil, how much flow vs pressure you need and what is priority. Clearly oils first job is to lubricate as this stops parts coming into contact with each other. Its other important job is to remove heat that is generated from stressing components like valve springs. Each time they compress they generate heat, oil is there to help remove this heat and stop the springs from thermal overload.

In a drag engine bearing clearance is usually loose. Loose is fast as it reduces rotational friction. Also oil temperature is usually much lower and the engine temperature is also much lower. so oil is naturally thicker even at the same viscosity. This does increase surface pressure tho. as surface area goes down surface pressure goes up. so bearing life can be limited. Refer to diagram 1. 

Another important area oil can work its magic is on controlling piston temperature. Japanese engines and some aftermarket blocks use oil squirters to help pull heat out of the pistons and control the growth of the pistons itself.  This is why you can normally run a tighter bore clearance with piston oil squirters. As you can see the job requirement for oil is more then just lubrication.

Diagram 2 is a great example of how precise bearing clearances need to be, too tight and the bearings limit flow, too loose and they can tear up.  For more information I have added a few papers on oils and bearings. 

Diagram 2
Here is some great articles and technical papers
Bearing Articles