[HummerJim]

The 08 engine runs on regular and it won't void your warranty or cause damage because of the engine computer controls, but it idles rougher with regular (just watch the tach bounce around), and the 6 speed will shift A LOT more on hilly terrain or doing any towing running on regular. To be honest, I haven't noticed much of a difference in mileage running regular vs. premium, but premium is "recommended" not required. I talked with an '09 owner at Lynch's Hummer outing this year and he said he didn't notice any difference using E85 ethanol or premium except for the mileage, but notices a "roughness" with regular.