93 octane.Car runs much better and have better fuel efficiency.I also liked alot the new Shell it seems different, I don't know if it's the specific gas station but is very noticable difference any time I use Shell 93.Car runs even smoother and has better acceleration.
If you use an octane too high for your car, you will have unburned hydrocarbons in your system which could lead to more carbon deposits. better to stick to 87 octane unless you are running a higher compression or FI.
I used 87 in my Contour except for autocross days - then to 93 w/ an octane booster too - don't really think it helped, but I THOUGHT it did!
Now in the Focus I use 87 - tried 89 and 93 on a recent trip to Florida and noticed NO difference in gas mileage. For now I'll keep using the 87 until I install the JRSC and have to go up to 93 all of the time.
Octane alone, will not make your car run better, if it is designed to run on regular at stock advance setting. Purity of fuel can be an issue. Ford does recommend 91 for SVT's. The more advance you run, the more air/fuel you can get into cyclinder. Low octane burns so fast, you use less advance, so it doesn't ignite before you get TDC. Since higher octane takes more time to ignite, you can sneek more in before it lights up. But if you use higher octane than engine was designed for, it means some raw, unburnt fuel is going to sit around the combustion chamber or be pushed out exhuast..Race fuel in a stock engine with stock settings, usually runs worse and worse.
Old trick of adding moth balls to fuel tank was to make fuel burn slower(try lighting a moth ball, they don't burn well at all) Vapor of moth ball cooled air charge- because it burned so slowly, and "raised" octane. Look at a can of octane booster. It doesn't make fuel burn better, it burns more slowly.
Disclaimer: Not responsible for burns caused by trying to light moth balls.