There's not a whole lot to wrap your mind around Lewis, but you do have to get past the "legends" and "myths" that are rampant about motor oil. And understand that the oil your Grandfather used is nowhere near the same animal as a modern motor oil.
Some background: A modern motor oil is 70-80% oil (base stock) and the rest is additives. Additives include anti-wear, dispersant, viscosity index improver, anti-foam, corrosion inhibitor, detergent, friction modifier, antioxidant, and others. The larger the viscosity spread (i.e. a 5W-50 vs a 10W-30) the more additives required to achieve it, in this case viscosity index improvers or modifiers. What you think is oil is really a carefully formulated lubricant that has all the additives it needs to perform or exceed API specifications.
Enemies of motor oil are oxidation, heat, and shear...any of these can shorten the life of the oil significantly. Oxidation from piston blowby, combustion byproducts, fuel/coolant dilution, soot, etc. result in the formation of free radicals that attack the carbon chain inherent in the oil molecule. This puts larger demands of the detergents and when they deplete, the oil can no longer keep the engine clean. Excessive heat assists this breakdown process and will also cause excess viscosity loss or migration of a multigrade oil toward a single grade. Shear is a mechanical process where the oil is squeezed out between to mating surfaces...the cams, piston rings, and oil pump are the primary culprits. Basically shear results in a viscosity loss which can eventually affect the bearings, which rely on a hydrodynamic film. Two of the three can be owner improved by using a better oil (modern oils fit this bill) and control heat (use an oil cooler). Shear is controlled by the engine design engineers.
Most the stuff you hear about oil comes from long ago and by word of mouth...mechanics and parts guys being the #1 source. The oil changed...the info did not. There was a time when you needed to change your oil fairly frequently...it simply could not handle it. The additives were not as good and the base oil was not as highly refined. The engines didn't help either...looser tolerances caused shear which breaks oil down. Few cars had oil cooler either, further contributing to the problem. The 3000 mile OIC was thus born and has been preached ever since...especially by the fast oil change guys that want you spending $$$ in their shops every 3000 miles.
The Supra owners manual is a case in point for OIC...it recommends a 2,500 miles under severe (operating on rough muddy roads) conditions or 5,000 miles under normal conditions. Even 20 years ago the engineers figured out that motor oil could exceed the 3,000 miles preached by mechanics, parts guys and Jiffy Lube...and, that is using a conventional motor oil. Modern cars have bumped that interval higher to 7500 miles (or more). Of course, the Jiffy Lube guys are going to come up with all kind of reasons why you should ignore all that and change at 3,000 miles...the main reason is they want in your wallet.
Fast forward to today and you still see the same old crap...doesn't matter that a modern synthetic has detergents, antioxidants, viscosity improvers and friction modifiers that are superman compared to 20 years ago. A PAO or ester based synthetic (all with identical molecule chains) using these additives result in triple the life span. Conventional wisdom tells you if the oil is getting dark, it's dirty and needs to be changed...in reality the oil is doing it's job and it getting dark is proof of that.
To finally answer your question, the anti-wear additives change chemically as the oil is heat cycled. It makes it more effective as it bonds with the base oil and friction modifier additives. Wear metals in analysis drop over time as a result.
None of that matters to guys like Zazzn who relies on "respected members of the community" or "my shit hot mechanic" for advice on oil. The problem is none of these guys can back it up or tell you why they run this oil or that oil. You hear "I like nice clean oil"..."my mechanic told me"..."I feel like it's better". Never mind the facts, it's all about how you feel this oil is doing and it getting dark is all the proof you need to run out and change it.
My response is whatever blows your skirt, but when you hear me tell someone that they can take a PAO or ester based synthetic to 8,000 miles easy, that is based on average conditions, average maintenance, an average air/oil filter and a stock (or no oil cooler). These oils can (and do) well exceed that OIC...if you use a bypass oil filter, you can make the argument for never changing the oil and simply doing part drains and add make-up. Oil analysis has proved it time and again...I really don't care what the ignorant have to say.
Cliff Notes:
- Modern motor oils are an order of magnitude better than when most oil "myths" started
- Anti-wear additives become more effective as the oil is heat cycled
- The fast oil change shops keep the 3,000 mile OIC alive so they can get your $$$
- A good conventional motor oil will go 5,000 miles and a synthetic 8,000 miles easy under average conditions
- Most parts guys, mechanics and even some "respected members of the community" don't know their ass from a hole in the ground when it comes to oil
