A few months ago, I decided to go premium and at the time I thought there was a slight increase in performance, and maybe an extra 50k's per tank..
Neway, as the prices rose, I thought stuff it, so I went back to regular unleaded and I didn't notice a single difference at all. The slight increase in performance and economy continued after going back to the regular stuff..
Therefore, either premium unleaded on a EA 3.9 CFI either cleans the engine/fuel lines or i was under the influence of good ol' placebo..
I do infact believe that on newer cars, or cars with an upgraded computer management chip, that premium does improve performance noticebly, but I doubt it would make a difference on a 14 year old engine with 230,000k's on the clock.