Transmission oil and other gear-box lubricants can perform for YEARS, operating at high temperatures, WITHOUT "breaking down"..They remain serviceable because they are uncontaminated by combustion byproducts..
"Sludge" in crankcases results from water-vapor collecting in oil or from the oil being partially BURNED by collecting in pockets, hot-spots on top of the head with very little oil circulation and temperatures over 350 degrees..You can call that "breakdown" if you like..I call it poor engine design...Failure to change engine oil in a timely manner compounds these problems because the oil becomes so contaminated with combustion products it can no longer hold them in suspension..
"The Story Of Why Engine Oil Becomes Engine Sludge
By Don Fedak
Fresh engine oil is a clear, free-flowing liquid blend of base stock and additives that contains no fuel, water, coolant, dirt, or other contaminants. In engines that have failed prematurely, the oil has very often been transformed into a high viscosity deposit of brown or black goo, commonly referred to as "sludge."
When regular engine oil changes are neglected, normally free-flowing lubricating oil breaks down, becomes contaminated, ceases to flow, and is transformed into a thick soup of waste products. That’s when serious engine damage is imminent."
This writer feels that "breakdown" and "contaminated" are the same thing. They are not...
Way back, when multi-viscosity motor oil was first being developed, the "viscosity index improver" which transformed 5 weight oil into 10W-30 oil would indeed breakdown in a process called "shear stability" or lack thereof..These problems have been solved long ago and viscosity stability is no longer an issue...