[Uncle Bob]

Just some quick thoughts, nothing like a thorough discussion:
The crankcase functions, in part, like a septic tank. The primary reason to change crankcase oil is to get rid of accumulated contaminants. The notion of the oil "breaking down" is specious. It's a terminology that is not well defined, and as such will mean different things to different people. If we talk about the basic oil material itself, conventional oil rarely, if ever, breaks down (in a chemical sense), quality synthetics are even more stable. The molecules may be subjected to sheer, making smaller ones of the bigger ones, affecting viscosity, but unlikely with gear pumps. Heat may take a toll on some lighter molecules, "burning" them off, but that would cause the oil to thicken.

Back to the septic tank idea. The air filter keeps out rocks, birds, and little children, but other contaminants come about as a result of various chemical elements in the fuel as well as moisture in the air. Drain intervals would be best judged on operating conditions. SAE, for instance, defines stop and go driving as severe. Those of you who read manuals will note that there are often different intervals specified for various operating conditions. Unfortunately, many folks don't realize stop and go is more injurious to the engine than sustained, relative high speed. Stop and go relates in part as noted above by operating time as opposed to miles, but also, condensation is more likely to form because of lower relative operating temps.

There is reasonable rationalization for longer drain intervals today than say 30 or 40 years ago. In part it's better lubricant technology, but also, taking the lead out of gasoline did a lot to not only prolong exhaust system life, but also spark plug life (yes, better materials there have added life as well). Leaded fuels (more accurately chemicals added to lower the boiling point of lead) formed more acids that in part contaminated the oil. Also, fuels are more tightly regulated for sulphur content today (especially diesel fuel), so less acid and soot are produced. There's more but that should give those interested an idea.

Personally, given my belief in the septic tank analogy, I don't buy the usual reasons given for use of synthetic oils. The notion of longer drain intervals was originally used by synthetic marketers to rationalize the disparity in price. As use/production volumes have shifted toward synthetics the price differential has narrowed, so it's less of a motivator. Synthetics have particular advantage in low temperature start up conditions. Oil flow, particularly at start up, is critical. The SAE at one time published data supporting that approximately 80% of engine wear over the life of the "normally" operated passenger engine occured at startup. If you understand hydrodynamic lubrication it makes sense. A lighter viscosity oil, again at the lower operationg temp range, will also do it's part of the engine cooling better. And the lower fluid friction improves fuel economy. All of those things motivate recommendations from engine manufacturers more than the extended drain interval thing..........though that does enter into some marketing think. But synthetics have no better capability to hold contaminants than a conventional oil. And the additive systems for each type are effectively the same. Modern computer controlled fuel/spark and other operating perameters make for cleaner operating engines, and less contaminated oil. Older engines still have a greater relative probability of contaminating the oil, even with cleaner fuels.

Changing oil "too often" isn't necessarily a bad thing if disposed of correctly. I'm not sure that this applies to many of the quickie lube change places, but while the oil service is being performed additional inspections for leaks, belt and hose condition, undercarriage damage, tires, and so on, should be performed. Those inspections alone, irrespective of oil capabiltity, may be reason enough, particularly on higher mileage rigs, to do more frequent oil changes.