This is a Good Question to put out there . . .
In my local area, we have 3 choices at the Pumps ( now most have that " I did that " Sticker on 'em )
Regular 87 Octane
Mid Grade 89 Octane at .25 to .45 cents more per gallon than Regular
Premium 93 Octane at .50 to .90 cents more per gallon than Regular
" INFINITI recommends using fuel with at least 91 octane, so that your vehicle will function the way it was designed "
This is the official statement as published - with this said. . .
It seems that 87 & 89 are of not recommended choices, and 93 is overkill . . . LOL . . .
I experimented one time, and filled up with Regular, I felt a reduced performance factor when getting on the QX50 . . .
The VQ 3.7 VHR, it just didn't get as loud and angry as it usually does when the accelerator was floored . . .
When I got to half a Tank, I filled up with 93 Premium to get to balanced average of 91 octane sloshing around in the tank, and noticeably, that loud and angry engine roar returned to normal levels of what was expected when pushing the QX50 to it's limits
Personally, I feel like I'm doing the QX50 a disservice if I under Octane it . . .
The QX50 scoots me through traffic, and performs admirably in all situations during the commute, and on all trips taken
I will reward it back by only using only 93 Octane ~
If it now costs an Extra $8 per fill up compared to the extra $4 it did 18 months ago, well, that's the consequences of well . . you know . . .