burlmart - Normally I expect to get 11.5 to 12 mpg, but I drive about 5 mph below the limit. Did get as high as 15 mpg coming back from Las Vegas around Christmas time. We were traveling down hill (dropping in elevation), had a tail wind, fresh water and waste tanks empty (winterized), no air conditioning, no trailer, and cool damp weather. All the conditions were right for higher mpg. The 6.0 has a little more power, the 5.7 is adequate.
As for the oil change light - must be new for either the 6.0 or the year. A simple call to any Chevy dealer service department should give you the answer. My Mom's Buick had a reset button to push in the glove compartment.