This exact same argument has been argued a million times in the mod4.6 forums. The conclusion is always the same, people argue that premium is better than regular, or plus is the best or that premium will hurt the engine. The majority of the guys with 99-mid 01 cars that are N/A are running 87-89 octane. The guys with mid 01-04 cars that are N/A are generally running 89 octane fuel. Why the higher octane in the later SN95's? Because compression was raised slightly on those models and a slight ping is noticed with 87 octane. Seems as though this is what is happening with the 05's as well. If 89 fixed the problem, I would suggest running 89 on a stock tune.