fiery,
I think 22% is generaly considered the min oil percentage in plain bushed engines for long life.
chevy,
Lubrication requirements vary with design sizes of the shaft and its clearances. There must be sufficient oil flowto maintain a full bush to shaft fill otherwise the shaft will begin to go into an oscilatory rotation which tends to squeese out the remaining oil film and cause increased wear. Imagine a shaft in a bush with a good deal of clearance , on the start of rotation(without oil)the shaft will try to climb around the circumference of the bush(cyclic rotation) as this is a crankshaft with vertical forces being applied it will be hammered up and down from this cyclic rotation - not pretty. Now if sufficient oil flow is present to fill the gap the shaft will rotate pretty much in the centre of the bush and be in a smooth rotation.
A ball race tends to trap enough oil in the groove of the race with very little resupply nessesary.

So how much oil, well that depends on the design and construction but generally 22% some PB will get away with 15% and be ok , but which ones ?
Now the little end generaly requires less oil as the film isnt being spun out, even the PB little ends in BB outboards and the like are ok at very low oil levels.
Stewart