Cary,
Interesting that these engines pinged badly on regular gas - particularly the 90s Bimmer. Generally, modern engines will ping briefly, then the knock sensors do their job and retard timing a bit at a time until it goes away. These days, if you have a continuous ping problem then there's likely an issue with the engine electronics or the cylinders have enough deposits that a) the actual compression ratio is up, and/or 2) the thermal mass of the deposits is enough that ping still occurs even at the limit of the engine's correction range. These deposits generally come from an engine in a poor state of tune, or continous use of poor quality gas.
I got the same info you did from the guy who worked at the Chevron plant on the coast south of LAX regarding sharing of gas. If one refiner brand ran short of a particular grade, they all had an agreement to provide cover. And it jibes with the additives being a major differentiator as he mentioned. However, he felt there was a significant difference between the generic brands that took whatever they found on a price basis in that they met only the minimum standards. The major brands, on the other hand, went well beyond this so they could make claims like you're seeing with Chevron (trucked to Detroit so the Big 3 can use it for EPA certification) and Shell (provides higher MPG than other brands). They spend big bucks trying to develop brand equity, and with that much on the line, it would shock me (and the Federal Trade Comission, incidentally) if there were not material differences.
IdahoDoug
