Can I run Edge level 3 on 90 octane(all thats available in my area), instead of 91 without causing any harm? My altitude is at 4200 ft.

Also...

If I can run 90 octane/level 3, what is the lowest elevation I can use 90 octane?

All that is available to me is 86 - 88 - 90. So does this also mean that if I was running stock tune or level 1 or 2, that I could run 86 octane instead of 87?

Anyone personally done this?

 

I have done this. I didn't run into any problems iwth the timing. I even advanced my timing .5 with 90 on level 3. But if i were you i would costom tune level 3 down .5 in the timing and see how your truck reacts. Then maybe push a bit more. Just make sure its not knocking.