What Octane Should I Run In My Subaru Ascent or 2020+ Subaru Outback XT?

OCTANE…

Well, this topic has come up three times in a week. So, I figured I’d address it.

What octane should I run?

Folks, the Ascent and 2020+ Outback XT require 87 octane (or higher) in all climates, at any altitude. If you live at sea level like me, it’s 87 octane (or higher). If you live at the top of Loveland Pass (highest road pass in the US at 11,990 feet above sea level), you still need 87 octane (or higher).

Some website/my Great Uncle Joe says 85 in the mountains…

No, run 87 octane (or higher). They’re wrong.

Why can’t I run 85 in the mountains?

A turbo is able to pull in considerably more oxygen than a carbureted (naturally aspirated) vehicle, thus, requiring the octane specified in the manual (87 in our case). Using 85 octane means the computer is working extra hard to try to prevent knock, and, more often than it should, will fail. Knock slowly damages an engine over time – especially a small displacement engine moving such a heavy load as the Ascent.
All of this is in section 7.1 of your Owner’s Manual.

On a turbo, for engine longevity, it is vital to use the specified octane(s), regardless of elevation.

In our case, that’s 87 octane (or higher).

Absolutely anything else is false.

What does the EPA say?

(short version: the same thing I just did)

What is 85 octane, and is it safe to use in my vehicle?

The sale of 85 octane fuel was originally allowed in high-elevation regions—where the barometric pressure is lower—because it was cheaper and because most carbureted engines tolerated it fairly well. This is not true for modern gasoline engines. So, unless you have an older vehicle with a carbureted engine, you should use the manufacturer-recommended fuel for your vehicle, even where 85 octane fuel is available. 

https://www.fueleconomy.gov/feg/octane.shtml

Should I run 91 or 93 octane?

What’s your reasons?

  • If for performance…
    No. The Ascent is not a dual performance tune car, nor a variable performance tune car. It’s performance tuned for 87 octane, and using anything else will not see a performance increase.
  • If for detergent amounts…
    Sure. For instance, Shell 93 has almost twice the detergent as their 87.

So, you decide if running 91 or 93 is right for you. But don’t run anything less than 87.

NOTE:

It is not advisable to run Shell 100/Shell 110.
It is almost always LEADED gasoline and will damage your emissions systems (eg: catalytic converter).

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