Here is what I understand about the situation. the 5.7L if equipped with the automatic recommends but does not require 89 octane, the manual, however recommends premium. What this means is you will have higher output if you use premium or mid-grade on the respective versions because the knock sensors have to pull things back a bit if you only run 87 octane. I have also heard though that it is not a wise idea in the long term to use 87 in engines that recommend premium or mid-grade. Although the computers are supposed to prevent denotation, computers can make mistakes. I would bite the bullet and just put in what the manufacture recommends.

I think the SRT engine requires premium.