Buying a car with an engine that requires 91 octane fuel and then putting 87 octane in it is stupid.
When you put less than the required octane fuel in a car, the performance and mileage go down the toilet because the ECU has to adjust the ignition timing to compensate for this and the result is very non-optimal timing. But the ECU has no choice becuase you put the cheap stuff in and it has to roll back the timing to keep the engine from breaking apart from knock.
Net result? You lose power. You lose mileage. You lose money.
If you put the **PROPER** fuel in your car you maintain performance, mileage, and efficiency. And because you do this your ECU lets the engine run with optimal settings where it's designed to run and the most efficient.
There have been many MANY debates on this over at Maxima.org, and the consensus is this. You can put 87 octane in the VQ3x, but you're better off just putting in 91 because if you put in 87 your performance and mileage go right down the toilet to the point where it would have been cheaper for you to just put in 91 in the first place.
So if you think you're saving a couple of bucks by putting in 87 you're probably going to find out you're wrong.
Just put in 91
If you're concerned about operational costs, maybe you should have gotten the 2.5 instead?? Or how bout an Accord V6?

The Accord V6 is the only mid-sized import V6 that was designed specifically to run on 87 octane fuel. This is one of the reasons I got it since I drive 30,000 miles per year. Being able to put in 87 saves me about $300/yr.
But don't put 87 in a car designed for 91. You'll just do more harm than good. And you're not saving any money either because your efficiency/mileage/performance will all go right down the toilet.