I have been lurking around this site for a while, and decided to register and make my first post on my impressions of these products. I bought Auto-RX and FP/LC because testimonials on this site, and am running both in a 2000 GMC Sonoma 4 cyl, a 1995 Chevy C2500 350 V-8 truck, and a 1982 F-250 Ford truck with a 351 V8. After 2/3 of the "clean" cycle of ARX, I notice little or no change in mileage or in how the Sonoma runs. The C2500 "feels" noticeable less harsh while running, and feels very smooth. I have not checked the mileage as this is a ranch truck and spends a lot of time off road and idling. With the 1982 F-250, after about 1/3 of the "clean" cycle, the truck was very, very less harsh, and very smooth and quiet when running. Each of these vehicles is on about the fourth tank of FP. I know my findings are not scientific, but which of these products would be showing the most effect? I know for testing purposes that I shouldnt run both products at the same time, but wanted the benefits of both and didnt want to wait.