I don't think that we need any additives for our cars but I do have to disagree with most of the posts here and I'll explain why.
I have an '04 Harley RG that's bored, stroked, high compression heads and because of all this it has a tuner (or else the stock ECM wouldn't run the bike). Living in the wonderful Commonwealth of Mass we have typically 3 grades of gas, regular at 87 (my Focus loves this thank goodness), a mid grade of 90 and premium at 93 (which is what my bike HAS to have. Over the summer I met up with a bunch of friends who live in Maine. We went for a LLLLOOOOONNNNNGGGGGG ride way up in the boonies one weekend and all I could find was a premium of 90 or 91. Talk about running like crap and pinging, I thought I would ruin the engine! I found a NAPA (I think) and bought 3 bottles of STP for the weekend. Made all the difference in the world.
My "other" car is a '04 350Z which also only likes hi test. I've played with this one regarding the octane boost and while it doesn't like 87 with the booster it will run fine on 90 with it (Dollar Store, $12.00 for a case of 9 too).
Ford Focus 2012 Hatchback 400A Titanium w/ Moonroof
Red Candy Metallic