Well, at least it was then, on a 2004 Chevy Silverado 5.3. GM was huge back then on lowest cost per mile and reducing environmental impact.So you proved that the OLM is correct, Conventional oil can go alot of miles, and the 3,000 mile oil change is a waste....
Not too many years after that, and on various engines, the OLM maximum was reduced by various amounts.
It could have been done for lots of reasons like squeezing more HP out of a certain displacement engine, turbocharging, more stress on timing chains, maybe even lower quality timing chains, who knows.
I think the only special GM spec back then was GM 4064 or something like that. Low temp pumpability I think it was. And conventional SL I think. It's been a long time.
At any rate, I still believe in the GM logarithm for oil degradation, as long as oil specs are met.
For the Mercedes, which is a simple "day counter", not so much. Even though most oils spec'd for those vehicles will last the whole year.