Re: oil, gas options?
Am constantly reminded of what Will Rogers said around 1935: "The problem's not what people don't know, it's what people know that just ain't so."
The owner's manual does not require "premium", it requires "91 octane" and includes a graphic showing they are referring to PON and not Research or Motor method.
As to why, the answer is probably the same as for what oil to use, EPA (or European equivalent) testing. To be accurate they should have also said "91 octane below 2,000 feet and above 80F" but that would have required a chart.