Rod bearing oiling issues are many-fold, beyond any problems that might occur with low oil level in the sump which could cause the pickup to be uncovered, resulting in aeration of the oil. Aerated oil spells the death of shear film bearings.
1. The mains see oil at the oil pump output pressure and flow. Main bearing oiling is generally way past optimal, unless there is a major flow or pressure problem in the oiling system.
2. The rods get "used" oil, which has been in the mains and has picked up heat, so it's viscosity is decreased; this is the beginning of the problems.
3. The rods orbit the crank axis, and the oil flowing from the mains to the rods is subject to (reactive) centrifugal force, so the "effective oil pressure" in the rods increases as the square of the crank RPM. You might think this is a good thing, but it's not, because...
4. The leak rate of a shear film bearing, like a rod bearing, is directly proportional to the pressure in the bearing. Now go back and re-read number 3: The oil pressure in the rods is going up as the square of the crank RPM, the leak rate from the rods goes up linearly with pressure, and the oil has lost viscosity in the mains. All this means that the rod bearing leak rate is going up in a big hurry as RPM increases.
This starts to look disastrous pretty quickly, as at high RPM, the oil flow from mains to rods can't keep up with the rod leak rate, and the rods run dry. We all know what happens next.
