Switching gas?

anytime, it won't hurt to mix them

 

Use "Regular". Whatever that IS when you fill up.
And don't worry about it.

As long as you will still be at a high elevation for most of the tank, 86 or even 85 will be fine.

You should go back to 87 as soon as you "come down out of the mountains".

 

In this instance, probably not the worst advice. With carbureted engines, lower octane helped vehicles run smoothly at altitude. The electronic engine controls that have replaced carburetors make the lower octane unnecessary. While the oil companies love charging normal prices for less octane, I think it was South Dakota that passed a law permitting 86 to be sold at elevation but making it illegal to sell in the plains.

 

Whoa, no technical terms!

A higher octane than recommended will not screw with an engine...ever. The only thing it will screw with is your wallet.

Long term use of a lower octane could potentially could screw with the engine, under the right circumstances and if there is a full moon, depending on how you define screw. The computers in modern cars will mostly adjust for any deviation in the recommended octane, although high performance could suffer.

Since you used a higher octane, you have nothing to worry about.

 

Exactly like Johnny Cakes said. Lower octane than the factory recommends can be bad. Higher will never hurt anything except your wallet, although it usually is no better than the factory recommended number.

 

Lower octane is never necessary. At high altitude, it becomes acceptable.

As soon as you find it available.

 

Well put. My wording was beyond inartful.