For anecdotal reference, this is an intake valve off my 2008 G8 GT. I removed it in 2013 when the car had 135k miles (redid entire top end for a cam swap).
I cleaned it with some gasoline and paper towel, then some soapy water. There was no real "residue" or anything on it previously (I display it on my office desk, so I didn't want it smelling).
I always used Sams, Shell, Racetrac, or whoever had the cheapest gas. Never was picky. Ultimately, I don't think it all matters that much.
