I have run 87, 89 and 91 octane in my 2004 V8LS here in SoCal.
I have had noticable pinging under hard accleration with 87, and good performance with maybe once in a great while ping on 89, and fine with 91.
A lot would depend on temperature and air conditions plus how you drive.
If you drive like a granny with mostly freeway driving on flat land, you could probably run just about anything for some time. If you are a hard driver in hot weather in lots of hills, you would need to move higher in octane.
For a little while you are probably OK with 87, I prefer 89. Best bet is 91 but you have a problem finding it.
Note that most Octane Boosters I have seen have fine print that says you have to use several cans per tankfull to move the octane any significant amount.
If it was me I would drive with 89 and forget booster. If I had to, I would use 87 without booster and drive gently.
Some people would swear your engine will grenade if you use anything less than 91. I tend to disagree since I have seen articles over the years about this and I would hope Ford would make an engine that isn't so delicate that it blows up if you look at it wrong. I could be wrong. Plus I have been running 89 pretty frequently since about April and haven't exploded anything, yet. Course the manual Recommends 91. Not the same as required.
Good Luck,
Jim Henderson