There are several factors to consider when determining bearing clearances. Detailed calculations can fairly accurately determine with the clearances should be.
The following are but a few:
1) The oil viscosity (higher viscosity i.e. 50W requires more clearance, 5W20 requires less clearance). The oil has to escape or it will overheat, but it also has to not squish out easily under the load. The old "rule of thumb" was to run high viscosity oil in hi-perf. engines. This does not hold true anymore with the advent of synthetic oils that can withstand high stresses with low viscosity.
2) The amount of distortion the in the bearing bore during operation (i.e. the rod bearing journal bore will distort more at 8000 rpms than at 4000 rpms). The clearance changes as distortion occurs.
3) The shape/concentricity of the bearing (i.e. a high performance bearing will be thinner at the parting line than a stock OEM bearing). The clearance is usually checked 90 degrees from the parting line. The high perf. bearing can withstand more distortion of the journal.
4) In the case of main bearings, the alignment of the main bearing journals in the block. I have seen ~0.001" misalignment occur from a bare block sitting on the floor to a block with the heads bolted on attached to an engine stand. The more mis-alignment, the more clearance is required.
5) The "twist" and wobble/flex in the crankshaft (usually only a concern with high hp/cid engines with high cylinder pressures).
In summary, if you are going to operate the engine in the same fashion as stock you can use stock clearances and OEM recommended oil. If you are going to use the bearings under extreme rpm/load conditions where bore distortions will occur, you should use more clearance. The Clevite hi-perf. bearings may recommend more clearance than the Clevite OEM replacement bearings.
Also, the OEM's have fractional oversized and undersized bearings and can control bearing clearances to about +/- 0.0003". When you get a crank turned and/or rods reconditioned you are lucky to get +/- 0.001". The replacement bearing manufacturer has to account for this amount of variation and will side on the side of safety (excess bearing clearance, rather than inadequate bearing clearance).
I have personally run main bearing clearances down to ~0.016" and rod bearing clearance (aluminum rods) down to ~0.018" on a 12,000 rpm engine and 0W (zero weight) racing oil with no signs of bearing overheating. I don't recommend that you run clearance quite this tight.
Also, engine bearing do not require a "break-in". The "break-in" only refers to the cylinder bores (which are not round in a new engine, especially an OEM engine) becoming round as the rings (which rotate and are made of much harder material than the cylinders) wear the cylinders round.
I hope this helps.
Greenlight