Just curious on the numerous "Fuel System Additives" on the market today. Are they worth anything? My friend swears by the Prestone Complete Fuel System Cleaner (for use every 4,000 miles). I am coming up on 6k miles for my '04 V-6 4x4 and was wondering about it...Is it worth the 4-5 bucks? Could it actually do harm to my beloved Taco? Any input is appreciated. Thanks.