Based on that, I would consider any towing or high altitude operation to be "severe duty". That's my read on it anyway.

What we have done several times is start with a 1/4 tank of 87, and topped it up with 3/4 tank of premium before we tow. That gets the octane above 87 with a 3:1 mix of premium/regular. 

We do have the 36 gallon tank, so a full load of premium is an impressive number here in California. We have dropped over $125 on one fillup.

QUOTE - GlueGuy   We use 87 most of the time, and put in premium when we are towing. The engine does produce more power when a premium fuel is used.

Do you use premium for towing all the time or have you ever towed using the 87 octane.  Was looking in the owners manual and didn't clearly state whether or not it is mandatory to use higher octane when towing. 

We have almost 50,000 miles on ours, and it has been great.

I can't speak for the F150's 3.5L Ecoboost, but I did have the 2016 Escape with the 2L Ecoboost. It was a fantastic engine. At any altitude above sea level, it had more power than the 4L V6 my current Frontier has.

That being said, my owner's manual specified that I could run 87 octane for general use, but said I needed to use premium when towing. I recommend checking your owner's manual to see what Ford says.

For naturally aspirated engines, a lower octane can be used at higher elevations as the thinner air works to decrease knock possibility. However, from what I have read, the turbocharged engines don't have this as the air they are stuffing in makes it more like sea level. The bottom line is that you should not go below Ford's recommendation as specified in your manual.

I recently traded in an older F-150 to an newer used 2017 F-150 which has the 3.5L Ecoboost.  Been reading about fuel and although it will not accept flex-fuel which my older 5.0L coyote engine would it states to put in no less than 87 octane.  Reading on a Ford forum I have read of some owners stating use nothing less than 90-91 octane (better performance/mileage) and others who live in higher elevations over 5000 ft. say they use 85 octane with no pre-ignition or other issues, even when towing.  I may be a bit cautious on this staying with the books recommendations of 87 only knowing that to rebuild the engine would be some serious $$$$.  Those of you with the ecoboost, what have you used when towing in higher elevations?  Thanks