I have some real world feedback on this.
My wife has been away for a few weeks, so I have been driving the 200. After reading all the gas threads here lately, I changed from premium to regular fuel, and have been running the roads hard. Have run 4 tanks through it.
Guess what...steering wheel resonance was virtually eliminated. It's still there, but it's faint and not really noticeable.
To test the theory I went back with premium yesterday...and the resonance picked back up.
Nothing else has changed with the truck. Temps outside are constant. Type of driving is identical.
I can't really come up with a reason as to why this is happening, but it definitely is...