You do realize that you can drive cross country and back and still not hit 6K miles right? Additionally, Hertz can put you on the
Do Not Rent (DNR) list if you drive more than 3,500 in a single month. Do you really think you'll hit 8K miles or more?
I'd return the car for an oil change at 7K miles or sooner if the oil change light comes on before you hit that.