First, to my knowledge there is no 'legal' definition of synthetic when it comes to oil. (I'm surpised no one is marketing an 'organic' lube oil to the easily-confused public.)
There are two types of processes that produce lubricant base stock, extractive and reactive. Crude oil is first distilled at atmospheric pressure into fractions of varying boiling ranges; these are mostly fuel components. What's left is distilled under vacuum, which is where the lubes come from. These fractions are then sent off for other extractive processes that remove wax, and other dissolved impurities.
So far, all we've done is purify the various fractions. Every molecule in the tank is unchanged from what it was when it was pulled out of the ground. So far, it's all been just a high-tech sorting activity.
You could make the case that once you apply a reactive process, the oil is synthetic to some degree. Even a technology as ancient as acid-treating would produce a 'synthetic' oil.
As any number of folks have pointed out earlier in this string, what matters is performance. It's entirely possible that an oil made with a high-quality Grp III will outperform an oil made with a crappy PAO. (Yes, there are crappy PAOs out there.)
Roger M - the reasons jet aircraft use polyol ester based lubes for the engine and phosphate esters for the hydraulics are fundamentally economic, but on a macro scale. They are not just considering operating and maintenance costs, but also the costs incurred when planes fall from the sky. If I use a cheap engine lube & my engine seizes up prematurely, I'll coast over to the shoulder and call AAA. The process is a little different in a 747.
We must also be mindful of your 'horses for courses' analogy. A jet aircraft turbine is a much different animal than a reciprocating automotive engine. PAO-based oils are already overkill for the great majority of automotive applications.
Even if a suitable polyol ester-based engine lube were available, I doubt it would have a favorable cost/benefit ratio when compared to modern conventional engine oils. While a jet aircraft turbine has a lot of problems to solve, piston ring blow-by is not one of them. Part of the job of automotive engine oil is to handle the crud that sneaks into the crackcase from the combustion chamber. Because of this blow-by, there is are limitations of how long you can extend the drain interval in this application.