There seems to be quite an amount of confusion over whether or not it's necessary to follow the manuals and fill up with 87 gasolene even at an elevation where 86 and 85 petrol is being sold as Regular.
When the discussion comes to whether there's any benefit to use Midgrade/Plus 89 or Premium/Supreme 91 in place of Regular 87, when the car manufacturer specifies 87, the consensus is that going with the higher graded fuel is simply a waste of money in most circumstances.
However, when it comes to 85 and 86, the picture is not as clear cut. Some claim that there was a major benefit of 85/86 many decades ago, whereas modern cars don't get any actual benefit. Others claim that it's actually detrimental for modern cars to not have the spec'ed 87 — the computer could still adjust, but you're going to lose fuel economy and power in the end.
What's going on?