You guys are all off. There is no particular restriction on fuel octane.
I remember an article in AutoWeek about this time it happened. It was really about the demand for premium unleaded and that the most common definition of "premium" was for 91 (R+M)/2 octane. Before then 92 was the most common. The old Unocal did have a patent on making higher octane rated fuel. I specifically remember old 76 stations selling a However, Unocal sold off all of its refining operations to Tosco, and that eventually went to Conoco before the ConocoPhillips merger and now Phillips 66. Not sure where the patent was assigned. When Chevron bought out Unocal, they were basically only a natural gas exploration company. And Sinopec tried to buy them first before US regulators tried to stop the sale on the premise that a Chinese company would divert an American division's fuel to China or perhaps use it against American interests.
By now that patent should have expired. One can find fuel that's blended with CARB-compliant 100 octane race fuel. That's basically all that one can find in California to make anything higher than 91 octane these days. But limiting conventional premium to 91 does make things easier for the refiners.
Found the article in AutoWeek. I don't remember they had it before but I had a print subscription back then. Apparently they're archiving a lot of their older print-only articles. The part on MTBE being pretty much gone wouldn't apply today since ethanol is almost universally added as an octane booster.
https://autoweek.com/article/car-news/no-more-92-premium-octane-rating-drops-california
This is another take:
Quote
http://www.superstreetonline.com/features/news/0102scc-technobabble
Whose fault is it this time, CARB? The EPA? The CHP? None of the above. This time we're being victimized partly by the oil companies, and partly--this is the one that hurts--by ourselves.
You see, when crude oil is refined into gasoline, the refinery doesn't have all that much control over what comes out. Crude oil is full of all kinds of stuff, and a refinery simply separates it, sorting all the iso-this and hepta-that in order of density. The really heavy stuff, like tar, is near the bottom, while the really light stuff, like butane, is near the top.
Somewhere in the upper ranges of the stack are the components of gasoline. There are between 10 and 15 different blend stocks, each with a different octane rating, which are mixed together to make gasoline.
The crude oil being used and little else determine the amount of each blend stock available for mixing. Generally, if you just dump all the blend stocks into a bucket, you end up with something around 88 or 89 octane. If you're selective and only mix the good stuff, you can make 92, 93 or even 95 octane. But once you take out the good stuff, you're left with crap--something like 85 octane. Then you have to leave enough good stuff in the bucket to bring this pee-water up to at least 87 octane. This limits the amount of 95-octane gas you can make. If you make 93-octane premium instead, you use up less of the high-octane stocks, allowing you to make a higher proportion of premium fuel.
In the Midwest, where an extensive customer base of good old boys in pickup trucks consume vast quantities of 87 octane, demand for premium fuel is low enough to make genuine high-octane premium.
In California, however, Lexus-driving executives suck down premium fuel like it's Evian, so 92 was the rule.
CARB isn't entirely innocent. Many of its standards for evaporative emissions and misdirected attempts at oxygenation have raised the manufacturing cost of high-octane gas, but it doesn't seem to be behind the sudden change to 91. Instead, according my super-secret oil industry mole, it all comes back to money. Unocal, you see, has a patent on the 173 easiest ways to make California-friendly 92-octane gas. As a result, every other oil company has to pay Unocal 5.75 cents for every gallon they make using one of these techniques. They haven't actually been paying it, but that's an issue for the lawyers to sort out.