G'day Musky.
Yes you're right. There needs to be a gap between the crank and bush for expansion and also to allow an oil film to be maintained between the bush face and the crank cheek. As you say too wide a gap offers the chance for oil to escape.
Depending on source, anything from zilch to 3 thou on a cold motor is what folks try for. The beauty of the NUP design is that you set this gap as specified in the usual way with the usual shims. The bearing restricts axial movement to that within its manufacture, once the cush nut is tightened, there is little float but a running clearance remains between crank and bush face because that has been set as per normal practice with the shims between inner race and crank. On and off of the inner race for shim selection is far easier, no rollers in the way.
The only downside is the cost, and the need to ensure the cush nut remains tightened to the high torque figure.
Cheers.
Swarfy.